Ok,
A bit of a hassle but very little cost, go and buy a new battery for your motherboard. Boot to your setup screen (Press Del) (BIOS) and set the BIOS to load best default or something like that, save and exit. Your PC should then boot without a problem.
If you are going to reinstall, follow these instructions from a windows site.
To install Windows XP to a new folder (also known as a parallel installation), follow these steps:
Start your computer from the Windows XP CD (or boot disks). To do this, insert the Windows XP CD into your CD or DVD drive, and then restart your computer.
When the "Press any key to boot from CD" message appears on the screen, press any key to start the computer from the Windows XP CD.
At the Welcome to Setup screen, press ENTER to begin Windows XP Setup.
Read the Microsoft Software License Terms, and then press F8.
Select the partition in which you want to install Windows XP, and then press ENTER.
Select the Leave the current file system intact (no changes) option, and then press ENTER to continue.
Press ESC to install to a different folder. (
http://support.microsoft.com/kb/316941)