I'm really frustrated because my PC is fairly fast and last night and this morning it's been acting really weird. First thing I noticed was that the sound was really choppy in Bioshock and it was lagging to the point where it was unplayable. I got out and went to Windows Media Player to look at some game videos I had recorded and it was lagging horribly and making weird sounds. I tried other games like World at War and Assassins Creed, the time it took for the game to actually appear from the time I clicked it was like a minute compared to 3 seconds or so before. When I finally get into the games, the load times are super long!

I had my CPU OC'd to 3.1GHz from 2.6 and it was running fine for a long time, so i resetted everything to default on the BIOS. Now I'm running everything at stock speeds and I still have the problem.
Someone please help me, any suggestions would be great.

XFX GeForce 8200 Motherboard
3GB Ram 800MHz
AMD Athlon64 X2 5000+ @2.6GHz (Had it at 3.1GHz)
Galaxy Geforce 9600 GT
Seagate Barracuda 40Gb and 500GB

How didi it get started?

I diconnected my 22' monitor to set up an HDTV Tuner. I put the tuner within inches of my open case. When I set the computer back up on my monitor everything was like that. I hope I didn't mess up my mobo or cpu.

Forgot to mention that when I turn my PC on, it says "CMOS Checksum Bad" probably really important. It never said that before.

Possibly time to reset the CMOS chip. It obviously has a data error in it, now, hence the checksum error [the data bits don't add up to what they used to, as a check]. So note the settings, then turn off the sysand pull the battery from the mb. Then either keep it out for several minutes, or use the jumper[ battery out!!].

Reset CMOS as advised . Also check for firmware software may be that one tried to install upgrade to CMOS and it failed. Try to to upgrade yourself your CMOS software from official manufacturer site of your motherboard.

CAUTION: Upgrading CMOS is very dangerous, make sure you follow carefully the instructions

Thanks a lot guys. I got the message to go away.
My computer is still slower than normal though.

I was trying to help you with this post of yours "Forgot to mention that when I turn my PC on, it says "CMOS Checksum Bad" probably really important. It never said that before." I thought my reply was germane...? If you wish for further explanation, please say so. No-one means for you to go away with your problem unsolved. A slow pc is not fun.
Anyway, you must elaborate a little: " Windows Media Player to look at some game videos I had recorded and it was lagging horribly and making weird sounds." What weird sounds? Were you running these from an optical drive or a hard disk? If optical, were the weird sounds from the drive itself, or from the speakers? Check Task Manager to see if anothe process is stealing a lot of CPU time... say, 30, 50%...
You disconnected a monitor.... I hope the sys was off if not an HDMI connection.
"I disconnected my 22' monitor to set up an HDTV Tuner." Set up the tuner to your sys, or to the monitor?
We can guess.. but it is not a great use of time. Tell us more.

First of all, Gerbil, that was a genuine thanks (not a sarcastic one).
I should have worded it differently.
Playing vids from HDD.
The PC was on when I disconnected monitor (DVI) and i set it up to my monitor only.
I've been getting random restarts today too. Looked into it and I'm working on seeing what's causing it.