This sounds very similar to what had been happening to my PC (I had XP Home, though). And I don't have a clear idea of why exactly it happened, either.
My PC would also just randomly start lagging, freezing, and the programs that connect to the Internet would just not respond. It seems that for me it also started often when I clicked on some system tray icon - or maybe I just noticed it there first, I don't know -, so the system tray and taskbar often froze completely too. And of course, the only way I could get out of this situation was resetting the PC.
I don't have any dodgy programs or viruses etc on my PC either, but I suspected this freezing happened because my Windows installation was over three years old, and as I've heard, they don't work that well after some time. About five days ago I formatted and reinstalled XP (Pro version this time), and now I'm waiting to see if the freezing will occur again. So far it hasn't, but I guess it's too early to tell. I'm sorry that I can't be of much help other than saying you're not the only one with a freezing problem...
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