HP Vista Computer locked up and now runs slow

We have a 3.5ghz HP desktop system with Vista Home Basic on it. It started locking up, so I did a system restore until I found a point where it would run again. However it runs so slow it might as well not be running. If you put it in safe mode it runs fine--but no printer or speakers of course--nor will it download MS updates. I tried doing the Windows Update thing in normal mode, but it just locks up. I've done everything I know to do (system restore and updates) but no luck. Any thoughts?
p.s. I have an older HP computer, 3.0ghz, Vista Home Basic, and it works just fine. Never slows down, never locks up. They're both on a home wireless network, which also seems to work fine.
Keith

 

What antimalware programs have you run so far?

 

Well, I couldn't run anything since it crawled and then locked up. I just kept trying "system restore" with different dates and suddenly it took and worked, started running fast and then automatically ran Windows Update and now all is OK. I then ran Avast! and SpyBot--didn't find anything.
Keith