To help narrow down where to look, please answer the following:
What has slowed down? Please describe how you are measuring performance (nothing fancy, just need a feel of what is going on).
Are you attempting to overclock anything anything? If so, what?
Did you install the latest GPU (a.k.a. graphics card) drivers from ati.com? If not, go to the link below. Download, install, and check performance again.http://sites.amd.com/us/game/downloads/Pages/radeon_win7-64.aspx
I recommend you install the Catalyst Software Suite.
Please clarify. Good luck!