Unplug your computer and pull the CMOS battery out of your motherboard. Let it sit for around 15 minutes and then try to start up your computer. If it boots up you can shut down your computer and put the battery back in.
^ What he said. What that does is clear the CMOS, which will clear all BIOS settings, which resets all clock speeds to default. About your video card, though: my personal preference is to either use MSI Afterburner or ATItool to overclock my video cards. They've both worked for me, but ATItool usually only works in certain scenarios. They're software video card overclocking utilities, so if you OC it too high, you get a system crash but you don't have to take anything apart to get it stable again

. Then once you have a speed that you think is stable, save a profile for that clock speed (you'll see what I mean if you use the program.) Also, if you have an presumably stable clock speed, run 3dmark03 or 3dmark06 afterward to confirm stability. Along with this, when overclocking, make sure to at least run ATItool's artifact scanner, if not along with MSI Kombustor, to test for artifacts: any signs that your clock speed is too high. Here are the download links:
ATItool:
http://www.techpowerup.com/downloads/436/ATITool_0.26.html
MSI Afterburner:
http://event.msi.com/vga/afterburner/download.htm
I've also seen software CPU overclocking utilities, but I haven't had much experience with them.
EDIT: Forgot to mention: if you use software overclocking utilities and set them to boot your maximum clock speed with Windows, and it turns out that the speed was too high and Windows crashes on bootup, you can always go into safe mode and set it to not boot with Windows.
