Try running this program: http://www.passmark.com/products/pt.htm
The average G3D mark of your videocard is 1953, definitely a powerful card. You can compare your rating to see if it's a system wide decline in performance.
Running the test gave some strange results:
At first 1610 w. 240 MHz GPU clock
Then
worse with 1155 at 500 MHz
Then a bit better with 1515 at 700 MHz
When I tried putting the clock at 850 the computer immediately crashed (black screen, graphics card fan at maximum).
Keep in mind that this changing of the clock speed was done in
ATI tray tools which is a few years old, but has never failed me before, and has some really useful features. I uninstalled it, however, to see if that changed anything, and it didn't, but I cannot get the Catalyst Control Centre to actually change my clock speed (neither after uninstalling ATI tray tools or before)
I'm thinking something really strange is going on here, there's no reason to the test results. I'll do a full virus scan overnight, they tend to take a while.
Here's a screenshot of the ATI overdrive screen:
When I change the clock speed and press the apply button nothing happens.
As for what I did when al this happened, the only thing that stands out is that I reinstalled Left 4 Dead through steam, since it didn't work after formatting the system drive (I left the data partition alone when installing windows 7). It was also when playing that that I first discovered the lowered FPS.