Explorer.exe crashes, no access safe mode

I'm starting here - you can move this thread later if necessary.

Sis-in-law called tonight in a panic, her computer has crashed. Was fine yesterday until she realized her firewall was off. She turned the firewall on, left the computer, came back some hours later and while she was using the computer Explorer (.exe not IE) vanished. She rebooted by using the task manager to shut down the computer. When she reached the desktop again, she turned off the firewall and rebooted thinking something with firewall was maybe causing a problem (being the only change to the computer in weeks). Now the computer boots, her desktop appears, then explorer craps out immediately. I tried to get her into Safe Mode and explorer fails there, too. Even when using Task Manager to execute apps, we don't have networking (despite selecting safe mode w/net). Not sure what to do here since she doesn't have anyway for me to help her get mal/spyware apps - and I can't be certain that's actually what's causing the problem...

HALP!:confused