Sounds like a Hackintosh as all Macs will load the current drivers for them via the OS X software update feature.
If that is the case, no help will be found here.
If you are setting up Boot Camp, just go to ati.com and download the file. Go here:http://support.amd.com/us/gpudownload/windows/Pages/radeonmob_win7-64.aspx#2
Be sure to go to the Individual Downloads tab and get that file and install it. Note that is for the 64-bit version of Windows. If you need the 32-bit version, get it here:http://support.amd.com/us/gpudownload/windows/Pages/radeonmob_win7-32.aspx#2