Check what other processes are running on your computer if you suspect a virus check for anything that looks suspicious. Otherwise clean your hdd ccleaner is a great tool if you haven't got it already also a defrag can go a long way in improving Hard disk drive performance.
Go to your power options and check that you (or someone else) hasn't accidentally enabled the power-saving mode.
Go to your power options and check that you (or someone else) hasn't accidentally enabled the power-saving mode.
Nice pickup hacakapelite, it totally slipped my mind that windows 7 auto enables power saving mode to balanced when you first install windows 7 it could have been running in that mode since you built the computer.