The last driver released for the Mustek 1200 UB Plus was for Windows 98 / ME / 2000 / XP.
There have been no new drivers released since then. See this: http://www.mustek.com/mustek/supportdrivers.php
The only way you can use that scanner on Windows 7 is by installing trial version of VueScan from here: http://www.hamrick.com/
If you want to keep using VueScan after 30 days trial period, it will cost you $39.95 for a Serial Number.
So your choice is clear: you either buy VueScan or buy a new scanner.
i downloaded a driver for the ES Windows 7 here http://mustek.com/mustek/Support.php --> http://mustek.com/mustek/supportdrivers.php
for ScanExpress A3 USB 1200 Pro USB Port Scanner
it works with photoshop. it didn't start the mustek software, but i'm not sure i installed it right, i forgot to have the scanner unplugged during install, anyway i was using photoshop to run it in xp too
the driver is dated Feb 2011