I can give you a quote of something I posted back in late 2009 that will explain how to fix an mbr trashed by the GRUB installer. That is from: "Oh No GRUB Trashed My Windows 7 MBR!" - Windows - Digg
Here I ended up simply unplugging the Host drive with 7 on it for a custom install on a second and later decided to install Linux on a usb flash drive and simply opt to press the F12 key at post time to either select the second HD or flash drive to boot into the other OS(Knoppix Live, ubuntu, Linux Mint Debian)'.
By isolation of the main drive you serve two purposes of 1) preserving the Windows mbr and 2) have each OS running as a stand alone. A third option is running any Linux distro on a VM.
Running the Startup Repair from the 7 dvd a few times(2-3 tiimes) while booted live from that can also correct a trashed mbr.
So what are the actual options to see Grub removed once and for all to see 7 startup normally once again after a Grub install has been seen? When booting from the 7 dvd we take a look at a different option also seen in the repair tools section seen under the large "Install Now" button following the initial language selection screen as the installer's setup options are first appearing. The command prompt is where you proceed to rather then the startup repair to open that up and enter these two commands at the command prompt itself.
First:type "bootsect /nt60 C:" just the way you see it typed there only without the quotation marks.
(Note the "bootsect" is followed by a space so you don't end up with "bootsect/nt60 C:" or two spaces instead seeing "bootsect /nt60 C:".)
Once you see the "successfull written" message appear onscreen you know that was entered in correctly.
The second command is one even more familiar: "bootrec /fixmbr".
This can also be followed with the second command: "bootrec /fixboot".
Upon seeing the "sucessfully written" message one more or two more times if you enter both commands following the first you should see a normal 7 startup return. You may also now start seeing "Windows 7(recovered)" as a start up item. That however can be renamed with another free tool available which we will come to in a minute.