You can get a disk for about $20 by providing proof of your ownership of the WinXP OS. You can do that by taking a photograph of the Windows sticker on the computer case, or by just taking the computer into a repair shop and asking them to make a Windows XP SP2 disk for you.
If you do take it to a shop, you could ask them to run a Windows Repair on the system if you would rather have them do this for you.
See the pictures at the bottom of this page.
You could also try to start the computer in Safe Mode:
Start the computer and at the first screen image, repeatedly press and release the F8 key.
When you see the Windows Start menu, scroll to Safe Mode and press the Enter key.
Choose your normal logon and hopefully the system will start in Safe Mode.
Once it has started, try restarting it to see if it will boot up in Normal Mode.