A while ago, I swapped laptops with my wife. She was running Windows XP, which was easy enough to reformat and install Debian onto. My laptop was running a dual boot Windows 7 / Debian setup. I wanted to keep the same Windows 7 on there (it was a Thinkpad with all sorts of utilities I didn't want to have to reinstall). So I needed a way to remove the Debian and resize the Windows 7 partition.
My first step was to remove the Grub bootloader. After a few quick searches, I discovered booting into the Windows 7 installer and running the command:
bootsect /nt60 SYS /mbr
was supposed to do the trick.
Unfortunately, after running the command, the laptop would no longer boot. I booted a GRML CD and restored the Grub mbr by running:
grub-install --root-directory=/mnt/sda3 /dev/sda
Afterwards I could once again boot Windows and Debian using grub.
After much head scratching, I finally found out the 'boot' flag for the windows partition was not set. Apparently this doesn't bother Grub, but prevented Windows from booting by itself.
After that, the rest was easy. I used GParted to remove the Debian partitions and resize the Windows 7 partition to be the whole drive, and everything just worked.