This problem is a known fault of Microsoft's SQL Server Express SP3 installer. It does not effect all computers. To ensure this does not happen to you, please make sure your Windows XP computer has service pack 3 (for windows) installed before installing SQL Express with SP3 in it.
If you have already experienced this issue, you can log into the computer using safe mode and then use system retore to return your computer to a point in time before the installation took place.