Dual Core problem

Could be a power supply issue. Maybe your PSU can't feed the chip enough juice. Go into your bios and check the voltages, or download a program called speedfan and see what your voltages are looking like.

Few things:

Did you upgrade to a new motherboard? If not - you may consider flashing the BIOS to the latest version. Also, if your system clock isn't keeping time, it sounds like the small button-cell battery on your motherboard is dying.

Dual-core system... did you install the dual core optimizer driver? You can get it here along with some other useful tools:

http://www.amd.com/us-en/Processors/TechnicalResources/...

Did you format and reinstall windows? If you changed motherboards, it is best to do a complete reinstall, then install the motherboard drivers. If you only swapped out the processor, you may or may not have problems, but a format/reinstall probably isn't a bad idea. If nothing else, make sure to install the dual-core optimizer driver.

i bet you didn't reinstall windows  

if you did, then i would check for a bios update

Not really a lot to add that hasn't been said. Start with a clean install of windows, make sure you install any applicable windows drivers for your motherboard and video cards, preferably downloaded from the manufacturer's websites. Make sure you're using the latest BIOS update for your motherboard. Also, be sure to install the AMD dual core optimizer as qwertycopter recommended.

Also noticed that you upgraded from 2GB to 4GB. Did you get identical RAM for the upgrade, or did you mix and match? Not that I think mixing and matching would simply slow you down, more a matter of blue screening, but it could be something to look into.

Thank you for the replies, I'm sorry I didn't include more information.

I did use a new motherboard, which is M2N32-SLI Deluxe. I did reinstall Windows, and I have not tried the AMD Dual-Core Optimizer - I shall try that now.  

Unfortunately, the Optimizer didn't seem to do anything. I installed it, and ran the .exe, but it didn't seem to do anything. It didn't appear in the processes either - am I doing something wrong?

What about drivers for the motherboard and video cards? Make sure that you're using the latest versions from the manufacturers websites. Motherboard drivers are critical! Also, with the video card drivers make sure you're installing 2 instances one for each card.

The Forceware drivers came directly from nVidia's website, and the drivers for the motherboard came from the CD included with the motherboard.

Check the motherboard manufacturer's website for more recent drivers.

Also, what about the RAM question I asked:

The 4GB of RAM came as a pack of four 1GB sticks. I'll try new drivers from their website later and see if that'll help. What about if it doesn't?