A client of mine's computer kept on installing Windows Updates on restart and shut down.
I Googled this problem, and rebuilt the Search Indexer, which didn't do the trick.
The computer also goes really slowly.
There are lots of errors in the Event Log (87 Critical between 7.43am and 8.43am today and over 4000 in the last 24 hours it reported - also reported as failed 3 Security Audits)
Plus:

ID 3090
Error
Application
Gatherer service

System
Servicing
ID's 4385, 4375
Error

and

System
ACPI
ID 6
Error

I have:

Thoroughly scanned it - however, McAfee said it was out-of-date, Fixed it to update it, then Windows reported it was still out-of-date, which it was (?!) - found nothing

Followed the D-A-L procedure of computer slowness

Spyware scan found 109 tracking cookies

MalwareBytes found Rogue.Installer, so I deleted that.

Restarted, crashed in the middle of the Dell initial startup screen.
Restarted again, started in Safe Mode, crashed.
Restarted again, started in Safe Mode, went to install Hijack This! from my stick drive, it said Application Not Recognised.
Restarted again, thought I'd see if it would start with Safe Mode with Networking, crashed.

Now the Vista screen loads and then goes black and crashes

Startup repair doesn't do the trick
Did Hard Disk tests and memory tests, all fine

All I can think of now is to update the BIOS

It's a Dell Dimension 5521, 1GB RAM, 160GB disk, Vista Home Premium SP1

Do you have Vista DVD?
BTW, that computer needs 1GB of RAM more.