Win 2k "Unknown Hard Error"

[dx2]

I've got a dell dimension 4300 that I just replaced the motherboard on and reinstalled the operating system(Win2k Pro) on a new hd and after about running for 15-30 min it just drops a box saying "unknown hard error".

Any ideas as to tracking down the cause of it?

EDIT:Tryin it with a new stick of ram. *fingers crossed*
EDIT:Rams not the issue. still got the error