There is no Windows 7 driver for the Mustek 1200UB as it is too old. You can either buy a new scanner or buy a copy of VueScan software from here: http://www.hamrick.com/
Download and install the free trial first and see if it works with your scanner. If it does, you must buy a serial number to keep using VueScan. Cost is $39.95 (US Dollars) for the Standard Edition. That's cheaper than a new scanner.