How to diagnose a suddenly slow Windows computer?

Ensign Taco writes: I'm sure nearly every one of us has had it happen. All of a sudden your Windows PC slows to a crawl for no apparent reason. Yeah, we all like Linux because it doesn't do annoying things like this, but the Windows desktop still reigns supreme in most managed LAN work environments. I'm running XP with 4G of RAM and a decent CPU, and everything was fine, until one day — it wasn't. I've run spybot, antivirus, and looked at proc explorer — no luck. There is no one offending, obvious process. It seems every process decides to spike at once at random intervals. So I'm wondering if there's a few /. wizards out there that know what to look at. Could this be a very clever virus that doesn't run as a process? Or could this just be some random application error that's causing bad behavior? I've encountered this a few times with Windows PCs, but the solution has always been to just add more hardware. Has anyone ever successfully diagnosed this kind of issue?

Questionable "anti virus"

[chilizard]

Yes several months ago a rootkit which was completely invisible to API appeared, I haven't heard any more but I'm sure we all will soon. You mentioned Anti virus and Spy bot is this a fix that suddenly popped up on the web with a warning of eminent danger? If so you have been victimized. Trust anyone that comes knocking on your display about as much as the guy selling timeshares in Venezuela. Before you get frantic about dying drives etc unplug the net cable or disable the wireless, reboot. note the res