Dual Booting problems with Ubuntu

I have been running XP and Ubuntu, dual booted from separate hard drives using GRUB2 boot loader. No problems.
I wiped XP and successfully installed Windows7.
A problem has arisen in that whilst GRUB still boots Ubuntu OK, when I select Windows7 from the GRUB memu I get the Windows option to 'fix start up problems' or 'start windows normally'. None work.
I am fairly sure I have some sort of MBR problem. Under XP I could get a terminal via the install disk and just "fixmbr" and all was well.
Following the same procedure in Windows7 gives me an X:> prompt. fixmbr doesnt work.
Can anyone advise please how to fix the MBR in Windows7 ?
Thanks.

All former bootrec and bootsect commands are automated in Win7 Startup Repair. You only have to run it enough times for it to attempt to repair and then rewrite the MBR - usually about 3 separate times with reboot.

I believe that dual booting Win7 with Ubuntu requires removing GRUB before Win7 install and then reinstalling it.

“How to” Dual boot Ubuntu and Windows 7 (Ubuntu installed first) - Ubuntu Forums

I suspect you are right about removing GRUB before installing - I didnt.
I will keep trying the startup repair but I havnt had any luck so far.
I am not that excited as it wouldbt be too bigger deal to delete both OS and do a reinstall of both - Windows7 first !
Thanks anyway.

I am a little unclear about what you have done so far.

You booted to the Win 7 Install DVD, selected your language, then Repair on the lower left and what happened after you did that? What did the message say?