I've tried everything I know how to do...HELP

My friend's girlfriend has a Dell inspiron I6400 running XP (I know sorry) with a T2080 processor at 1.73GHz with 1 GB of RAM. I've told her to invest in a new laptop but money is tight I think so she just wants this one fixed. Any help is much appreciated and I hope I give all the necessary info needed to do that.

She is completely unable to download anything. If she tries it will either bring her to a "page not found" or will just let her download and then the computer won't follow up and let her run the installer.

I've tried running in safe mode, running "rkill" (which are all system32 processes of either either rundll32.exe, runonce.exe or grpconv.exe) and then trying to install avast, hijack this and malwarebytes but to no avail. Each time I try to install one of these programs I get "The system administrator has made this action unavailable" even though I'm signed in as the admin with full priviliges.

I would really appreciate any help on the matter and I hope I've given everything relevant although I'm by no means good enough to know what you need!

Thanks

Very good chance that its a virus. Some virus are written just so you cant download and rid yourself of them. How about running an online scan. To check.
House Call is very good
HouseCall - Free Online Virus Scan - Trend Micro USA
Then for malware
http://www.superantispyware.com/onlinescan.html

Many times you will not be able to download in safe mode, try a clean boot
http://support.microsoft.com/kb/929135

Hi

As said by Rich your system has been infected by viruses and all of it's critical executables are infected as well. Do online scan and when everything gets done make sure you have shields for a battle ahead, if not you'll have to do a repair when core files fails to heal.

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these are just suggestions