We'll need some specifics about your box in order to hopefully help you with this issue.
You say you're running an Nvidia GF3 card right? Are you having these 'freezes' running the native drives or have you installed the Nvidia drivers?
What, precisely, is happening (i.e. does it 'freeze' on login to KDE/Gnome--with user input or no? It may be something as simple as a prob with permissions on something like a directory that you're trying to 'display' or otherwise.
Long and short of it though...we need more info