What is a cmos battery?
It is the only battery on the motherboard. It keeps the memory holding basic startup and config settings alive.
If you have an XP CD, boot off of that. Then run the Windows setup. Look for a screen that asks if you'd like to run a repair install of Windows. Not Recovery Console. Repair install. Then let the install run to completion. None of your data will be lost. It will only do an overwrite of Windows files. If this does not complete then it is likely your hard drive.
Next step would be to get a new hard drive, install it in place of your old one, place the old one as a slave unit then install Windows on the new one.
When next you get in to Windows, you will be able to browse your old drive to get all you data off of it. For each of these procedures you WILL need the 25 digit license key number that came with Windows or the machine. It is likely on a sticker on the outside of your PC.
If those steps do not help, then there is a hardware problem elsewhere. Probably the motherboard.