On your keyboard you probably have a key located between the CTRL and ALT keys that looks like a windows logo.  If you tap that and the letter E at the same time it opens up "my computer".  Many times I have been able to unfreeze my computer  without losing information and having to reboot by going in there to the program that is causing the problem (or to extra running programs that may be overloading the system causing the freeze) and closing it.