pentium fours have a bit of a fault where music production is concerned
something about the way they do their maths
it means that if you use effects with long reverb and delay tails it can make the processor fall over quite quickly , wish i could explain this more fully , but i cant remember exactly what it is , when i get home tonight i will find out and post a link to the info.

I have a friend who builds pc`s for studios , dont touch a pentium 4 was his advice , I have one though and mine indeed does fall over much quicker than it should .