My Computer keeps locking up with increased HD usage

[Nodoubts2k]

My Computer (Athlon XP 2700+, 512mb, XP PRO SP2) continuously locks up when I try to extract large archives (over 3gb), or even repair multipart archives using quickpar. I have no idea what I should do here, I have tried using it in another user mode (with less junk installed), and nothing. I've also done a virus scan and ad-aware, and still am lost as to what to do. Any help would be appreciated.