Random 'beep' freezes

Hello fellas,

3 year old pc, recently cleaned of all spyware no problems with it previously.
Last couple of weeks, if im running more than say two programs, theres a tendency (like once or twice a day now), when i try to select a window, the cursor moves but won't let me click anything. If i try to go down to the taskbar it just gets to the edge of the taskbar and the cursor turns into the one with the arrows on either end.

It normally gives me about 3/4 attempts at clicking on something and then beeps and at that point the cursor and machine just lock up. Unable to alt-ctrl-delete or even reboot on/off from the front of the tower. I'm forced to power down round the back and then reboot.

Any ideas? Hijack this isn't showing anything dodgy.

 

Also, when I reboot, about 3 or 4 times ive had a blue screen and it says something about firefox profiles, cache, first allocation is not valid entry will be truncated. Says this about 10-15 times down the screen for different numbered entries. Im baffled.