Before you do either of the ones above, try this (it's pretty quick and might just work).
Insert your Windows XP CD into the drive. Then open a command prompt window (start, (all) programs, accessories, command prompt). Then type this:
expand X:\I386\INTL.CP_ C:\WINDOWS\system32\intl.cpl
Where you replace X with the letter of your CD-ROM drive.
If it works, you should see a message telling you how much it expanded by.
This may or may not restore your regional and language options, depending on whether anything else is broken. If it still doesn't work try above poster's suggestions.