Assuming your Windows install was on sdb1
(the "EFI System Partition"), then I'd recommend you do two things;
Poweroff your system and disconnect sda
Boot off of your Windows media disk and go to the recovery console.
You'll want to use the Recovery console and issue a 'fixboot' and maybe a 'fixmbr'.3)
I strongly suggest you hit up Microsoft.com and read-up on those commands that pertain to whichever Windows distribution you're using.
Reboot and do a test-boot of your Windows distribution. If you got it working, then
Poweroff, reconnect sda
, then add an entry in your grub setup to boot sdb1