I've installed it on 64 bit W7, no idea whether it was a 32 or 64 version I installed though .
It would be helpful to know when you get that error.
My guess is that you need to uninstall it, but you will need the USB drive connected and using the same drive letter when it was installed. To check what drive letter it was installed to have a look at the properties of the shortcut on the start menu that launches Gimp, then make sure the USB drive is connected as that drive so the program can be uninstalled.