First off, go to GIMP.net
Find the gimp that that your computer needs, if you need gimp xp then download it.
DO NOT download the regular XP version, apparently it has an error with XP and you must do something else. This will be explained later.
Now, if you download the regular XP version, if you have an original no-counterfeit WXP, you will have to follow the other step of this guide.
If you have a non-counterfeit version, you will have an error while the GIMP file unzips itself in the document GIMP library.
Then, go back on the gimp website and download the windows xp PORTABLE version.
The portable version is for older computers that do not have the gimp file patch.
The GIMP should start up now, but it may not respond while in the middle of starting up.
This occurs when you must restart your computer to let gimp patch itself.
Than, gimp should work for you, enjoy your shop software!
For windows7, just do the same operation. Just download the WINDOWS 7 Portable Version.
Rep + if i helped.