Computer running slow all of a sudden

Totally like Linux since it doesn't do annoying things like this, nonetheless the Windows desktop still rules supreme in most managed LAN labor environments. I'm running XP with 4G of RAM and a decent CPU, plus everything was fine, until. It wasn't. I've run Spybot, antibug, and watched at price explorer. There is no one offending, palpable procedure process decides to speak at once at random recesses. So I'm wondering if there's a scarcity wizard out there that know what to look at. Could this be a very clever bug? Or could this just be some random application blunder that's instigating bad behavior?.
