|louisp50, a couple of things come to mind. Your problem could be either/neither, but then neither will do any harm, so...|
First is a file called 'index.dat'. It gets read every time and can take quite a while if it’s too bloated. It's like a log file and keeps a record of every site you visit on the web. Empty, it's 32K, but it can grow HUGE. It's a protected file and can't be deleted by normal methods. CCleaner (free @: http://www.ccleaner.com/) can get rid of it when it’s ticked (not to worry, it gets rebuilt EMPTY on the next boot.).
Next is a worm/trojan. Be aware that a virus and a Trojan are similar, but not the same. Many times, an AV detects one it can't clean, other times, it gets missed entirely.
If you need to clean one, Superantispyware can do the job and you can get it free @:
among other places. They have a Vista compatible version if needed.
If it turns out that you need to use it, be sure to update prior to running and disable 'restore' (restart after cleaning) so it doesn't get put back. They just released a new update recently and it changes all the time. Things are in a constant state of flux.
The restore thing is critical. May not be your deal at all, but won't hurt anything to try.
Ed in Texas.