Are you getting the blue screen of death?
Do you have a Linksys network card?
If so, it's the newest service pack that has compatability issues with your NIC card. You have to manually install previous drivers for the NIC card before it works properly. I had to deal with this on my mom's PC. It's a new PC and a new NIC card, but it just randomly started acting up.. low and behold the PC did an automatic update and would own itself.