Slow Computer

I just built a new computer w/ 4 mgs of ram, 2.67 ghz duo cpu, 6 hard drives. all over 300 mgs of memory. the main 'c' drive is 500mgs of memory all HDs are SATA. 'C' drive was brand new and freash copy of XP2nd editon. graphics card is a g-froce 6600

Problem: computer runs fine until you ask it to do mutlipy tasks or the same task over and over again..ie..opening different photos, or running different movies, then the thing starts to slow down so much that the movies skip, they will stop then go forward then stop...

I thought the duo core was billed as being faster, yet I feel I took a step backwards..

I take it it's a new install of XP as well?