I had Vista set up in Bootcamp previously and ran into problems getting the drivers and Catalyst Control Center to install as well. With the default Bootcamp drivers, I was experiencing major graphic corruption in Windows and needed to update the drivers.
So, I downloaded the ATI Radeon HD 4800 Series drivers and installed, but still had the corruption. Poking around, I discovered that only the Catalyst Download Manager had been installed, not the drivers or anything else. So, after several attempts with no success, the light bulb finally went off.
I remembered that the iMac is basically a really big notebook in a fancy case. So, I downloaded the ATI Mobility Radeon HD 4800 Series drivers and gave them a go. The GPU was detected correctly and the drivers installed and no more screen corruption or video driver crashes.
You just have to make sure you get the correct drivers, since the iMac uses the Radeon Mobility GPU....I assume so anyway. But, that's what worked for me to get the ATI drivers installed in Vista under Bootcamp.