Refuses to Update.

I wrote on a previous topic that I grabbed two cheap sets of HP Vista business 32 bit off Ebay.
I installed one of them to one computer which has an ECS motherboard. All was well except for a Hard disc thrash which I eventually tracked down with the help of this forum, to a Readyboost/Superfetch clash. I then verified Vista and installed SP2.
I was impressed enough with this setup that I decided to install the second set onto my wifes comp which had Win7 64 bit RC1 on it. It has a Gigabyte M/B.
I installed it over the Win7 and all went well until the last screen which has the very thin progress indicator on the bottom and the words, Checking 3d etc. The installation hung, I left it for an hour and then pressed the restart button. I then discovered that the MBR had been corrupted and neither Windows or Linux( Gpart ) could see the HD. I got it working by using Nuke and Boot and reinstalled Xp and then loaded Vista over that. Again Vista got to the same screen and hung. I had to Nuke and Boot again.
This time I loaded XP to a tiny Partition and installed Vista to its own large Partition and got through to the same screen and then got a BSD.( 1st one I have seen in years ).
The comp rebooted itself(?) and Vista booted. Readyboost was working on this machine, Great! One prob it refused to shut down, I left the comp running for 24 hours and after that ran Ccleaner/Diskcleaner/Defrag. Since then it hasn't had a prob except I noticed that its not updating. Autoupdate is selected. I DL SP2 and tried to install it, no go.
I went online and verified Vista and that worked Ok. But no updates, I even went to Microsoft and tried to update from there. Nothing.
Sorry this is so long, but any help would be appreciated.

In my opinion it is any hardware issue here;diagnose your RAM by using memtest Memtest86+ - Advanced Memory Diagnostic Tool and hdd by using manufacturer tool Bootable Hard Drive Diagnostics - Vox

If you have access to system find all DMP filles in C:\Windows\Minidump and attache to post

Flaviuos, Thankyou for your reply. I apologise for not replying sooner, I have been playing around with this Vista install/update prob.
As you suggested I ran a mem test for four hours and no probe showed up.
I then had to reinstall as a BSoD appeared and this led me to look at the difference between the two comps. The M/Bs are almost identical i.e. Nvidia chipset etc.
The only diff was the good install used a ps/2 mouse and the other was wireless.
I changed them over and the install went through perfectly, along with the updates.
So all is good and all down to a mouse.( I think the prob might have been a battery low on power).
Again, thankyou for your help.