I was wondering if anyone could suggest a good system diagnostics tool that can be d/l from the web. I need one that will run at boot as I can't get windows onto a computer I am trying to repair. Preferably something that stress tests the system as well. Thanks.

For Spyware protection and general help cleaning up the computer and registry I recommend, the following files, as I have found 1 or even 2 is not always enough as 1 may find something the others do not.

Spyware
Microsoft New SPyware program-Formerly Giant spyware, they bought it out.
Spysweeper
Spybot search and destroy
Adware SE
Aluria's Spyware Eliminator
Spyhunter

I would suggest at the very least 3 or more of the above programs.

Registry and Fix and Repair programs
Fix it Utilities 5
Advanced System Optimizer
Ace Utilities
System Mechanic 5

Any 2 or more of these.

And Google toolbar with built in pop up stopper for your browser is good.

And a good solid Anti Virus Program or 2, updated weekly.

A good defragging Program or 2
Diskeeper Lite
Perfect Disk.

Also run from Start/run type in chkdsk then let that run then type in chkdsk /f to help fix WinXP errors.

Also use the below website, that tests and reviews some for the best programs out for many different things, they give you side by side comparisons and rate each one in many categories.
http://www.toptenreviews.com/

Most antivirus will do that