Coming late at this, but just in case it helps in the future. The MBR is not corrupted when you overwrite it with the GRUB bootloader code, any more than overwriting it with the MSWindows bootloader code. Corruption implies damaged data. The GRUB MBR is not damaged at all in this case, but the Linux /boot partition would have been.
As has been already explained if the Linux partition (or boot partition if it is a multi-partition installation) is deleted GRUB will not be able to start up any OS, including MSWindows. This is because all the necessary fs drivers are in the linux /boot directory which was wiped out.
The solution is to run fixmbr from a MSWindows installation/recovery CD (
http://www.nu2.nu/pebuilder/ or similar should do the trick).
Alternatively, there are two immediate options that I can think of in the Linux world:
1. Create a back of your MBR.
It is prudent to plan in advance and create a back of your MSWindows MBR sector using a Linux LiveCD (sysrescuecd knoppix, etc).
Run this:
mount /dev/sdb1 /media/sdb1 <--mount a USB stick, or
mkdir /mnt/windows
mount -t ntfs-3g /dev/sda3 /mnt/windows <--mount your MSWindows partition
dd if=/dev/sda of=/mnt/windows/mbr_backup bs=512 count=1
umount -t ntfs-3g /dev/sda3
The above assumes that the main MSWindows OS is on /dev/sda3. Various OEMs have additional recovery and boot partitions these days. If you are not sure use gparted, to see what the labels of the partitions say and anyway you'll need to shrink the MSWindows partition before you install Linux.
When you remove your Linux OS, all you need to do is use a LiveCD and run dd in reverse:
mkdir /mnt/windows
mount -t ntfs-3g /dev/sda3 /mnt/windows <--mount your MSWindows partition
dd if=/mnt/windows/mbr_backup of=/dev/sda bs=512 count=1
umount -t ntfs-3g /dev/sda3
2. Instead of using GRUB to chainload the MSWindows OS, you can install the GRUB bootloading code in the Linux partition boot record and chainload the Linux OS from the MSWindows boot manager. The process is more convoluted because you will need to edit the MSWindows boot menu to point it to the Linux boot sector. I'll only give the highlights here, Google should help with the rest:
Install Linux and make sure that you ask the installation manager/script to install GRUB to the Linux partition *not* the disk. Then use dd (as I showed above) to make a back up of the Linux partition boot record and save it to your MSWindows partition. Then edit your MSWindows boot menu (MS Vista and Windows 7 have a different boot menu file than previous MSWindows OS' so you'll need to read up on this) and point it to the back up of the Linux boot record. That's all.
However, the catch with this is that if you ever reinstall Linux or install a new version of GRUB, then you'll need to repeat the above process or Linux will not boot with the old back up of the boot record. With a distro like Ubuntu which keeps updating the MBR this is a relatively regular process (say 3 - 5 times a year).
I'd recommend option 1.