Are You Getting: Stop 0×0000007B, Error in Windows XP

AHCI

AHCI is a setting for your SATA Hard Drive, Look in the BIOS and check if this setting has been changed

Driver Issues

Have you recently updated? Or updated device drivers for your computer. If so try pressing F8 while the computer is booting and select

Last Known Good Configuration

If the “last known good configuration option” doesn’t work then using F8 boot into safemode and remove any updates you did or roll back any drivers. Have a look at Microsofts article on rolling back drivers.
http://www.microsoft.com

Hardware Issues

Have you added any new hardware? If so remove this and see if Windows boots correctly

Hard Drive Corruption

If you hard drive is corrupted or more specifically your boot sector then this can cause an error. Bleeping computer have an indepth article on how to get into the Windows recovery console.
www.bleepingcomputer.com

When you get into the recovery console run the following commands to repair your hard disk after which reboot your computer

chkdsk /f /r

fixboot

fixmbr

Final Solution

If you can not get the above solutions to fix you problem then the last thing to try is a repair install. This will overwrite the Windows XP installation and move your files to a safe location

Use the following link for an excellent guide to repair install.
http://michaelstevenstech.com/XPrepairinstall.htm