Sudden extremely bad lag on my PC

This has been happening for a few days now. I can usually run any Steam game full resolution at medium settings and get around 30 FPS. However, for the past few days it's been really horrible. On TF2 now on absolute minimal settings, 640 x 480 with "very low" or "no" set on everything. I won't get any more than 10 FPS. And it isn't my internet connection either, all of my other offline games act in the exact same way. I've tried running a deep virus scan, nothing. Disk Cleanup, nothing. Disk Defragmenter, nothing.

This has completely ruined my gaming and I need to know why it's happening in the first place.

BTW, I've got a 2.79 GHz processor and 2 GB of ram. I've got an outdated graphics card, but that couldn't be the case because it's been working just fine till now.

Sounds like a hardware problem. Quite possibly with the graphics card. I dont' know.

Did you install any software recently that could be hogging system resources?

Try reinstalling drivers? It could also be a case of faulty hardware

full specs might help. (nvidia card or ATI or another one, core 2 duo or single core...)

Intel Pentium 4 CPU 2.80 GHz according to my properties. For a graphics card I've got a five years-outdated nVidia GeForce FX 5500. It's probably what's keeping me from going to higher settings, but I don't see how it could cause this sudden horrific lag.

try reseating your graphic card.

I'm thinking steam just decided to screw itself over, I'm just reinstalling it and the games and seeing if that works.

It didn't work. I just finished reinstalling Steam and it's even worse. TF2 lags at the MAIN MENU, and GMod is constantly freezing. Those games both take lots of resources, but I always close unnecessary programs before I play. I also pretty much removed half the programs on my computer to see if that was the cause.

I had the same sort of issues and replaced my graphics card and it worked. I also had quite a few textures not sticking though so you're situation isn't exactly what I was in.

Yeah, but since I'm 12 I still have to rely on my parents for everything, and I can't exactly earn the money to buy a new card.

did you try re-seating it like I said? there could also be something in your PC case blocking your fans or your cards.

I don't exactly know how or want to break inside my computer right this second. And how are you supposed to reset it?

no, not resetting, reSEATing.
see if it's sitting in a wrong position or if it moved a little by itself.

I'm too lazy to check right now, and I found a solution. You know those configs that make it look like a PS1 game?
http://img684.imageshack.us/img684/8862/tf2onps1.png
It works. It still won't go above 20 FPS but it is playable.