I've never heard of running A PC under a lot of stress for a while permanently slowing it down. But (and this might seem a bit far out there...) maybe you burned out one of your memory sticks.
How much
RAM is installed in your computer?
Please do the following:
Go to Start>>Control Panel>> Click on "System" (this is "Classic view", if this is set then you'll see a bunch of folders when you first open the Control Panel)
If it's not set to Classic view, then please do so by clicking the "Switch to Classic View" (this can be undone later, instructions below) link in the left menu and open "System", post what it says for memory. If that number is lower then what you put for my first question, memory is the culprit and you'll need to replace the bad stick(s).
To switch back from Classic view in the control panel, just click the link that says "Switch to category view", again on the left menu