As you can see in this link... http://www.mustek.com.tw/Download/driv_scanr/english/1200ub.html, Mustek does not apparently provide a Win7 driver for this model. You could inquire with their support service... http://www.mustek.com.tw/Support/ to see if they have something but just haven't posted it on line.
Your best hope might be if Win7 OS has include a generic driver for the scanner.
Have you checked your Device Manager to see if the scanner is listed?
if none of the above works, use the Vista drivers. During the beta & release canidate of win 7, there were some driver conflicts and the best resolution was to use the vista drivers as they were similar.
if you have win 7 64-bit, use the Vista 64-bit drivers, same goes for 32-bit...