I've notice that there is a hidden directory call IE7 in the windows directory which is only used when its installed, and MS decided not to tidy this up after the install. I've deleted it from a number of machines now without any side effects. how about adding it to CCleaner?
There are a couple of other directories I'm not sure about windows\installer, I think this has stuff in you download, windows tents to download stuff and then copy it to the required directory, and I don't think its cleaning up after itself.
And IE7Update needs to be cleared, MS in its wisdom stores the patches for IE7 in a different place to all the other patchs.