Recovering Ubuntu after installing Windows

Hi,

I would be grateful for some help with this:

I share a computer with someone else and have my own hard disc in it in addition to his which is devoted to Windows. I was asked to upgrade his Windows XP installation to 64-bit Windosw7.

I did this and all went well but I lost the ability to access Ubuntu which is on /dev/sdb1,(Windows is on /dev/sda1). I reinstalled GRUB but in a moment of distraction I reinstalled it on /dev/sda instead of /dev/sdb

Now when the machine starts I get the boot menu like I did before but if I select Windows there is the following message:

"Error No such device........"where the word device is followed by a 16 digit string. If I do nothing, after about 10 seconds it boots normally into Windows.

Can anyone suggest what I can do to get rid of the error message? Although it does not affect the functioning of the computer, it is causing a bit of irritation to its owner. I had thought of using the Windows install CD to fix Windows and then re-installing GRUB but I wondered if there was another way.

Any suggestions would be greatly appreciated.