My pc laggs

[ehallaby]

Hello,
I want to know why my pc is lagging, it runs well for 5 minutes then lags for 1 minutes runs well for 5minutes etc... Im running a Amd Athlon cpu 5000+,4gig ram,1tb harddisk with lots of space, video card,Geforce gts250 1gig and a 550W power supply with windows 7 ultimate

 

[sysdaemon]

define your "lag"