Think I had a virus or trojan

Mullinax: I've used CCleaner (the first C stands for crap, seriously).

pcryslerfat: That "Blue Screen" (a lighter blue than the BSOD) that fixes files is a check disk scan that was initiated automatically since Windows did not shut down properly. I recommend running that scan periodically, and it can be scheduled from the command line or via the hard disk properties sheet. Now when you slid the breaker to restart, was this in an off state, or was it powered on and frozen? I want to be certain I'm understanding the facts correctly. If it was off, I'm still betting on the power supply. BTW, HP uses proprietary power supplies, so that $30.00 ATX unit from Walmart or Newegg won't work. You have to get one designed for your specific configuration.

Dustermaniac: It sounds like your Hard Disk is degrading, and windows cannot read certain files. Nothing to panic about, a HD is an electromechanical device, and is prone to this. I have observed this on PCs as new as 3 months, but it typically happens after two years or so. From experiance, I'll tell you it's NOT just the indicated file that is missing. You could replace that one file, then another would be indicated, and so on. The first thing to do is a check disk scan with the /f /r switches. This will map out bad sectors so they are not written to again, and in the case of important system files, it will actually restore the corrupted files. If it does not restore the files, perform an "in place" install of your OS. Not all OEM media allows this, so be careful. It is easy to accidentally wipe your system while trying this, if you hit the wrong prompt. If your important files are not backed up, do that first.

I realize I'm not giving specific instructions on all the procedures I have listed. This is not to be a PITA. All the info is available for free if you Google it. Sometimes you just need to know what to google. I'm a network and systems admin, by trade. I deal with infrastructure and servers mostly. I really hate dealing with client machines, especially Windows, and extra especially Windows XP.