Which program did you use? Perhaps there is an option in it to revert changes.
Otherwise this may help. (Use at your own risk, backup your registry before making any changes, create a restore point):
Use Registry Editor (Regedit.exe) to follow these steps:
Create a DefaultIcon subkey in the following registry key, and specify a default value of "%1":
HKEY_CLASSES_ROOT\exefile
If the HKEY_CLASSES_ROOT\exefile\DefaultIcon registry key already exists but has a default value other than "%1" , change the default value to "%1".
Delete the following subkey from HKEY_CLASSES_ROOT\exefile\shellex:
IconHandler
Create a DefaultIcon subkey in the following registry key, and specify a default value of "%1"):
HKEY_CLASSES_ROOT\icofile
If the HKEY_CLASSES_ROOT\icofile\DefaultIcon registry key already exists but has a default value other than "%1", change the default value to "%1".
Delete the following subkey from HKEY_CLASSES_ROOT\icofile\shellex:
IconHandler
Close Registry Editor and restart your computer.
(source) The original article was for Win95, but I checked my registry and it seems that in this part it is still the same in XP.
Use the "%1" without quotes.