Start the computer in Safe Mode (repeatedly press the F8 key during the boot process which will take you to a boot menu where you choose Safe Mode) and choose the Administrator account to logon to. If you've never created a password for that main Administrator account (which in XP Home is hidden except in Safe Mode), after clicking on it to select it, press the Enter key.
Once in Safe Mode in the Administrator account, create a new user profile in Control Panel>User Accounts. giving it administrator privileges. Then reboot the computer into the new profile you just created.
If that works, the MS KB article
How to copy data from a corrupted user profile to a new profile has the instructions to get the new user account exactly the same as the old one which probably, as the title of the article implies, got corrupted in some manner.
Just leave the old profile in place until you're sure you have everything you need from it (a week or two is usually enough).