You have a choice of spending the time to be a computer repair person or try simple work arounds. I suggest you:
1. Put it on a laptop cooling pad.
2. Use the CPU speed controls to tell it to run at a lower clock rate.
This upsets some but I have a 6 year old laptop that runs fine once I did that. Cheaper than repair or replacement.
3. Do the scan in safe mode.
This will not fix issues, will not get around bad hard drives, corrupt hard drives, bad sectors or other bad hardware.
4. Unplug all USB devices, disconnect networks shares and printers.
There are options short of replacing the machine.
Recently my laptop has frozen and refused to complete an entire antivirus scan. I was using McAfee, but was advised to uninstall it and install Antivir instead, but this didn't solve the problem. I then had a new motherboard fitted, but this didn't solve the problem, so finally had the hard drive reformatted. However I'm still getting the same problem, although not as badly. The scan can't seem to proceed beyond a certain point, although it doesn't make all my other programmes go non-responsive anymore, and the computer clock doesn't stop at the time when the scan froze.
I'm running a Dell Inspiron 1525 Intel(R) Core(TM)2 Duo CPU T5750, with Vista SP2 and my browser is Firefox, although I've had the same problem running the scan with IE.