1. You must be able to boot to ?Safe Mode?, ?Safe Mode with Command Prompt?, or ?Last Known Good Configuration? to run System Restore which can put the Windows back to the time when it ran fine.
2. You boot to Recovery Console to uninstall Windows Service Pack 2 (SP2).
3. Reinstalling Windows on top of the current one will uninstall SP2. If your problem was caused by SP2, this process will make your Windows bootable. You have to reinstall all MS critical updates. Make sure you do not install SP2 again.
4. Clean installing Windows should work unless you install SP2 again.
You must identify the cause of the rebooting problem. The way I see it, SP2 caused Windows unstable because the motherboard (MB) BIOS is not compatible (or ready) to SP2. To fix this, you have to do BIOS flash to see if it works. Please provide detail of your MB Manufacturer (MSI, ASUS, ABIT, Intel, and so on) and Model.