The problem is that you're on 64 bit and drivers need to be signed. ATI dropped your card to legacy support, meaning there is no 64 bit driver on offer for Vista/7.
I can hack a driver for you but then you are going to have to permamently disable driver signing or hit f8 when starting the machine each time to choose to do so.
Things would be easier for you on 32 bit, but can't really do much about that now without troubles. Disabling driver enforcement permanently is rather easy to do though, so you don't have to hit f8.
Please read what I've said and decide how you'd like to do. If you want me to hack a driver for your 64 bit regarding the above circumstance, I will be right on it for you as soon as I ask a piece or two of info...