What am I overlooking?

I have a fresh install of Windows XP, and up to SP3. This computer is slooooow. I have a 3.4G processor, and 3 gig of DDR2 ram. If I have iTunes open, and try to start firefox, the whole system hiccups for ten seconds, well more accureately, the systems freezes for ten seconds or so, then goes on as if nothing happened. The Mobo is just shy of brand new, and I know the ram is good. Would a power supply cause this? Bad processor? I know for a fact there are no virus or malware of any kind, I use a thorough virus/spyware/malware program that I trust.
With the kind of processor speed, and amount of memory, I expected more out of this system.
Thanks for any suggestions, I'm stumped.
 
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