there is a virus that causes this, or a bad update...
The instructions to repair XP are posted at the top of this forum. Follow the directions to "repair XP."
Repairing does not erase your personal files, but it does set the operating system back to default.
If you do get the XP to repair, immediately back up all of your files, once the computer boots up again.
Problems with XP are better solved wiping the hard drive by reformatting.
Then install a fresh OS.
BUT backup your files FIRST before making a fresh installation.
OR you can have a computer shop do it for you, usually about $200.