Thanks for your advice Southern Lady, Booyah I may need to follow your steps later on but I think I am nearer to finding out what my problem is. It seems to be a hardware device problem. When the PC is freezing I hear this beep beep sound from inside the base unit. Sometimes when I am restarting it takes ages on the 'scanning devices' screen. I also got an error message telling me to check my devices. I dunno if this is connected but on Thursday(the day before these problems began) I plugged my printer into the back of the base unit. I hadn't had it in for a few weeks. I couldn't plug the printer into it's usual socket on the base unit cos I have a soundcard breakout box in there, so I just put it in a purple socket(quite high up on the back of the base unit) that it fit into. Could it be this socket is unsuitable for a printer?
Also, I did a panda online virus scan and before it froze up
it told me I had 13 infected files of which it only cleaned up 1. AVG and Housecall turned up nothing though. Really frustrated right now but I hope I am nearer to discovering the problem. Again, all help much appreciated with this headache.