Vista - Defender mayhem

I purchased a new desktop system from Dell a couple of weeks ago. I noticed
that it came with some junk preinstalled, so I proceeded to delete the Roxio
software, that had about 8 different entries in the installed software list.
As soon as I finished doing it, things started to go wrong:

-Everytime I turn on the computer, I get two error messages:
"MSASCUI.exe-Bad Image
C:\Program Files\Windows Defender\MpRTMon.DLL is either not desiged to run
on Windows or it contains an error. Try installing the program again"...etc.

"WMI Provider Host stopped working and was closed".

The second one is particularly insistent, and will pop up every so often.
As a result of whatever has happened, I can't access Defender or even
reinstall it (as the installer says it's already part of Vista). My antivirus
(AVG at the time) stopped working completely, and the Security Center has
also stopped responding at all. I'm concerned, because I now have no
antivirus protection at all.
Tried doing a quick reinstall of Vista, but it found an error and couldn't
be completed.
Any help would be much appreciated, since I'm at my wits end with this one.
-----------------Since I posted this, other programs have become unstable or
non responsive: Internet Explorer and Windows Live Messenger

Logfile of Trend Micro HijackThis v2.0.2
Scan saved at 9:43:15 AM, on 8/19/2007
Platform: Windows Vista (WinNT 6.00.1904)
MSIE: Internet Explorer v7.00 (7.00.6000.16512)
Boot mode: Normal

Running processes:
C:\Windows\system32\Dwm.exe
C:\Windows\Explorer.EXE
C:\Program Files\Windows Defender\MSASCui.exe
C:\Windows\System32\rundll32.exe
C:\Program Files\Creative\Sound Blaster X-Fi\Volume Panel\VolPanlu.exe
C:\Windows\System32\Ctxfihlp.exe
C:\Program Files\Common Files\InstallShield\UpdateService\issch.exe
C:\Windows\System32\rundll32.exe
C:\Program Files\Common Files\Real\Update_OB\realsched.exe
C:\Windows\ehome\ehtray.exe
C:\Program Files\Dell Support Center\bin\sprtcmd.exe
C:\Program Files\Digital Line Detect\DLG.exe
C:\Windows\ehome\ehmsas.exe
C:\Windows\System32\CTxfispi.exe
C:\Program Files\Mozilla Firefox\firefox.exe
C:\Program Files\MSN Messenger\msnmsgr.exe
C:\Program Files\Internet Explorer\IEUser.exe
C:\Users\Plectrum\Desktop\HiJackThis.exe

R1 - HKCU\Software\Microsoft\Internet Explorer\Main,Search Page =
http://go.microsoft.com/fwlink/?LinkId=54896
R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page =
http://www.finderg.com
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Default_Search_URL =
http://go.microsoft.com/fwlink/?LinkId=54896
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Search Page =
http://go.microsoft.com/fwlink/?LinkId=54896
R0 - HKLM\Software\Microsoft\Internet Explorer\Main,Start Page =
http://go.microsoft.com/fwlink/?LinkId=69157
R0 - HKLM\Software\Microsoft\Internet Explorer\Search,SearchAssistant =
R0 - HKLM\Software\Microsoft\Internet Explorer\Search,CustomizeSearch =
R0 - HKCU\Software\Microsoft\Internet Explorer\Toolbar,LinksFolderName =
O1 - Hosts: ::1 localhost
O2 - BHO: Adobe PDF Reader Link Helper -
{06849E9F-C8D7-4D59-B87D-784B7D6BE0B3} - C:\Program Files\Adobe\Acrobat
7.0\ActiveX\AcroIEHelper.dll
O2 - BHO: Skype add-on (mastermind) - {22BF413B-C6D2-4d91-82A9-A0F997BA588C}
- C:\Program Files\Skype\Toolbars\Internet Explorer\SkypeIEPlugin.dll
O2 - BHO: SSVHelper Class - {761497BB-D6F0-462C-B6EB-D4DAF1D92D43} -
c:\Program Files\Java\jre1.6.0\bin\ssv.dll
O2 - BHO: (no name) - {7E853D72-626A-48EC-A868-BA8D5E23E045} - (no file)
O2 - BHO: Browser Address Error Redirector -
{CA6319C0-31B7-401E-A518-A07C3DB8F777} - C:\Program Files\BAE\BAE.dll
O4 - HKLM\..\Run: [Windows Defender] %ProgramFiles%\Windows
Defender\MSASCui.exe -hide
O4 - HKLM\..\Run: [NvSvc] RUNDLL32.EXE
C:\Windows\system32\nvsvc.dll,nvsvcStart
O4 - HKLM\..\Run: [NvCplDaemon] RUNDLL32.EXE
C:\Windows\system32\NvCpl.dll,NvStartup
O4 - HKLM\..\Run: [NvMediaCenter] RUNDLL32.EXE
C:\Windows\system32\NvMcTray.dll,NvTaskbarInit
O4 - HKLM\..\Run: [VolPanel] "C:\Program Files\Creative\Sound Blaster
X-Fi\Volume Panel\VolPanlu.exe" /r
O4 - HKLM\..\Run: [CTxfiHlp] CTXFIHLP.EXE
O4 - HKLM\..\Run: [UpdReg] C:\Windows\UpdReg.EXE
O4 - HKLM\..\Run: [ISUSScheduler] "C:\Program Files\Common
Files\InstallShield\UpdateService\issch.exe" -start
O4 - HKLM\..\Run: [Google Desktop Search] "C:\Program Files\Google\Google
Desktop Search\GoogleDesktop.exe" /startup
O4 - HKLM\..\Run: [ISUSPM Startup]
C:\PROGRA~1\COMMON~1\INSTAL~1\UPDATE~1\ISUSPM.exe -startup
O4 - HKLM\..\Run: [HostManager] C:\Program Files\Common
Files\AOL\1186515444\ee\AOLSoftware.exe
O4 - HKLM\..\Run: [TkBellExe] "C:\Program Files\Common
Files\Real\Update_OB\realsched.exe" -osboot
O4 - HKCU\..\Run: [ehTray.exe] C:\Windows\ehome\ehTray.exe
O4 - HKCU\..\Run: [DellSupportCenter] "C:\Program Files\Dell Support
Center\bin\sprtcmd.exe" /P DellSupportCenter
O4 - HKUS\S-1-5-21-3744954288-2266759499-257931790-1000\..\Run: [ehTray.exe]
C:\Windows\ehome\ehTray.exe (User '?')
O4 - HKUS\S-1-5-21-3744954288-2266759499-257931790-1000\..\Run:
[DellSupportCenter] "C:\Program Files\Dell Support Center\bin\sprtcmd.exe" /P
DellSupportCenter (User '?')
O4 - Global Startup: Digital Line Detect.lnk = C:\Program Files\Digital Line
Detect\DLG.exe
O9 - Extra button: (no name) - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} -
c:\Program Files\Java\jre1.6.0\bin\npjpi160.dll
O9 - Extra 'Tools' menuitem: Sun Java Console -
{08B0E5C0-4FCB-11CF-AAA5-00401C608501} - c:\Program
Files\Java\jre1.6.0\bin\npjpi160.dll
O9 - Extra button: Skype - {77BF5300-1474-4EC7-9980-D32B190E9B07} -
C:\Program Files\Skype\Toolbars\Internet Explorer\SkypeIEPlugin.dll
O13 - Gopher Prefix:
O18 - Protocol: skype4com - {FFC8B962-9B40-4DFF-9458-1830C7DD7F5D} -
C:\Program Files\Google\Google Desktop Search\Plugins\gdSkype\skype4com.dll
(file missing)
O20 - AppInit_DLLs: C:\PROGRA~1\Google\GOOGLE~2\GOEC62~1.DLL
O23 - Service: AOL Connectivity Service (AOL ACS) - Unknown owner -
C:\Program Files\Common Files\AOL\ACS\AOLAcsd.exe (file missing)
O23 - Service: Symantec Lic NetConnect service (CLTNetCnService) - Unknown
owner - C:\Program Files\Common Files\Symantec Shared\ccSvcHst.exe (file
missing)
O23 - Service: DSBrokerService - Unknown owner - C:\Program
Files\DellSupport\brkrsvc.exe
O23 - Service: GoogleDesktopManager - Google - C:\Program
Files\Google\Google Desktop Search\GoogleDesktop.exe
O23 - Service: InstallDriver Table Manager (IDriverT) - Macrovision
Corporation - C:\Program Files\Common Files\InstallShield\Driver\1050\Intel
32\IDriverT.exe
O23 - Service: SupportSoft Sprocket Service (dellsupportcenter)
(sprtsvc_dellsupportcenter) - SupportSoft, Inc. - C:\Program Files\Dell
Support Center\bin\sprtsvc.exe
O23 - Service: stllssvr - Unknown owner - C:\Program Files\Common
Files\SureThing Shared\stllssvr.exe (file missing)
O23 - Service: XAudioService - Conexant Systems, Inc. -
C:\Windows\system32\DRIVERS\xaudio.exe