Print off this post before starting! Read it in full before starting!
To remove the overclock you need to go into the BIOS - usually by pressing Del at startup. The first thing to do is note down ALL the settings. On paper. You'll likely need to go through lots of sub-menus. This is a real chore, but will allow you to set things back as they were. If you take photos, print them out before continuing. Now go and have a cup of tea or coffee away from the PC. When you return, go through your notes and confirm against the screen that they are correct and that you haven't missed anything. The break is important as it means you're applying a fresh set of eyes to the task.
Assuming you're still satisfied, there should be an option in the BIOS labelled Reset to Default or similar. Select that, confirm, and reboot. If there isn't, use the Clear CMOS jumper or button on the motherboard.
When the PC reboots, go straightaway into the BIOS and set the date and time and check that the detected hard drives and boot order are correct - you noted them down earlier, didn't you?
Exit the BIOS and the PC should boot into Windows. If it doesn't, go back into the BIOS and check the settings against your notes.
