Xp sp3 Installation error; Computer will not boot.

Hello,

I have a problem with my computer. (obviously:laugh
After starting the installation of service pack 3 for windows XP, I get prompted many times that Access is denied to the registry. A short while later into the install, the installer crashes. I then get prompted with this message: Access Denied. I click ok, and a patch uninstaller comes up and removes the service pack files and registry keys. Now it prompts me to restart my computer. I do, and my computer will now not boot. I get as far as the option to enter the bios, but after that the screen turns black, and it stays there.

Im hoping this can be fixed without re-installing windows, somehow:4-dontkno....

If you know please post here!

Thanks in advance,
sky geek.