(Both of the following directories are hidden.)
For Windows XP, the directory to delete is C:\Documents and Settings\All Users\Application Data\Apple Computer\iTunes\SC Info\.
For Windows Vista/7, the keys are found at C:\ProgramData\Apple Computer\iTunes\SC Info\.
Then, you'll need to uninstall iTunes, since you need to downgrade to 10.0 (be sure to back up your iTunes library, in case it gets corrupted!). (
32-bit,
64-bit)
Once you have iTunes 10.0 installed, try to open a protected song. It should ask for your Apple ID and password, because it needs to fetch the keys again. When the song's playing, that means Requiem should be able to decrypt the song.
When the songs decrypt fine, you can upgrade once more to 10.1.1, and restore your iTunes library.
Hope I didn't lose you in that!
And before someone asks about it... Requiem does not remove the DRM found on IPAs (or rented videos).