Help! Why is my computer so painfully slow?

I am running Windows XP on a fairly newish (maybe 12 months old) Dell. If I reboot, it runs fine for a while then starts getting slower and slower. If I try to switch windows or even close one down, it chunters and chunters away but doesn’t do anything. I ran Disk Clean-up and it told me I didn’t need to Defrag. Any ideas?