Computer running like crap... Hardware tests?

I've just recently been noticing that my computer has been running pretty badly lately. Games that I used to be able to run flawlessly with settings all the way up (TF2, Dragon Age, even Batman: Arkham Asylum) now see lots of hitches and slowdowns.

Today, I bought Dead Rising 2 after hearing on the DR2 forums that people with almost the exact same build as me (nVidia GTS 250/9800 GTX+, Intel Core 2 Quad 2.66 GhZ CPU, 4 Gb DDR2 RAM) could run it pretty well with their settings all the way up. I played the game today only to find that when I got going, the game would slow down to about 5-10 frames per second if there was any amount of zombies on-screen.

What I really want to know is, is there any kind of test I can dowload and run my computer through to try and figure out if there is a single component that's failing for some reason? I really want to try and figure out if there's something I need to replace, but I can't think of what could be wrong. I know something has to be wrong because nothing has changed hardware-wise, and I just recently reinstalled a fresh copy of windows, so i don't think that could contribute to anything.

Well if you've just recently reinstalled Windows you might wanna check to see if you have the latest drivers for everything in your system. Also if you haven't already try doing a disk cleanup, and a disk defragment.

I agree. How much space do you actually have left on your HDD?

My HDD is only half filled. Also, trust me, I have drivers for everything. That's the first thing I do whenever I start getting problems. Disk cleanup says it can get rid of a whopping 116 mb and I have 0% fragmentation. I really doubt it's hard drive related.

I'm looking for tests, but I can't find anything that looks like what I'm looking for... Maybe it doesn't exist :/