I would recommend checking out this site http://www.cyberiapc.com/overclocking.htm it gives very easy to follow instructions on Overclocking your PC.If your Processor is underclocked then my guess would be the BUS sped it set incorrectly so simply setting it to a higher value should boost it up to full speed.

I'm not the most advanced PC user here by any means so perhaps wait a bit longer until one of the more advanced members shows up