Installed 12.04 on clean second (sdb) hard drive (windows 7 is sda). On first reboot I get Grub error 15 and system dies. On restart system always goes to Grub and now can't boot windows.
To boot windows, use your bios to boot directly from your windows drive, since they are two separate drives. The windows drive should contain an MBR on the root of the disk and should boot just fine. You can do this by changing your boot order, or by using a 'boot menu' if your bios supports it during boot.
Next, you can use a ubuntu livecd to recover grub on the second disk. Use the guide found on the ubuntu wiki to accomplish this: