Does for example <Alt>+<Tab> still work to switch between windows?
Or is the system completely frozen (can have many causes - even at hardware level)?
If the system is completely frozen, do you see the CAPS LOCK light blinking?
And last but not least:
If you experience such a 'freeze', does the following thing work:
<Ctrl><Alt><Printscreen/SysRq> and then, while these three buttons are pressed in, press and release the following buttons one after one:
R E I S U B
(wait at least about 1 second in between)
=> this is to try to do a safe rescue reboot (if you want more info about it: http://en.wikipedia.org/wiki/Magic_SysRq_key
(remark: some people say you don't need to press the Ctrl-button
If that doesn't work anymore, we would like to know also....
So, waiting now to the next freeze