ok - run regedit.
navigate to HKEY_LOCAL_MACHINE/SOFTWARE...
you should find a list of folders in here, pertaining to all the apps/games/progs,etc you have installed.
now look for folders Ubi Soft and UBISOFT.
depending on whether you have any other Ubisoft games installed, you need to delete any entries referring to IL2.
if you have no other Ubisoft products installed, you can delete the entire Ubi Soft and UBISOFT folders and their contents.
if you do have any other Ubisoft games, it's a bit more tricky, you need to go into the UBISOFT folder and delete any IL2 related subfolders.
once this is done, reboot your machine and you should be good to go for a fresh install.
any hassles, let us know - but REMEMBER - messing with regedit is potentially dangerous! make sure not to delete anything other than the IL2 related folders.
if in doubt, post here with questions, and we'll go through it - but you should manage with the info i've given.