One of my computers has been acting up. It's a Compaq Presario 5202HM running Vista. When it's being used, it often make a "whirring" noise that will persist for several minutes. Then it stops, and it's quiet again for awhile. Every once in a while, this will happen when a program (like the anti-virus) is updating, but it happens at other times, too. During the time when the whirring occurs, the computer's performance is extremely sluggish and it's almost impossible to do anything with it. It's defragmented weekly, with Disk Cleanup run at least daily. The diagnostic program that came with it doesn't come back with any evidence of hardware failure. Any ideas?