Weird lagging and skips

I've just finished putting together this comp from parts of old machines and everything works great except for one thing. At certain times the whole computer "stops" and is unresponsive to any keystrokes or mouse movement. It's the equivalent of "lag" in a game. It seems to happen when IE is loading images or a shockwave thing or when I start downloading something. What could be the problem? I had this cpu/mobo working fine in a previous config. Here's my setup:

P3 450 katmai
BCM QS440BX mobo
512mb PC133
Voodoo3 2000
13 gig western digital
Creative Soundblaster AWE 64 (ISA)
2x2x6 CDRW
Fresh reformat and install of Windows 2000 Pro (NTFS)

What could be the problem? I am thinking its the hard drivecause it is old and I havent use it forever. Would defragging help? Thanks.

try swapping a different network card, that's the problem my friend had when his computer was stuttering.

do u have dma enabled for the hard drive?