You will have to change the mbr to remove the pointer to grub. I use a
Knoppix CD for this. The procedure is to boot into the CD, bring up the
terminal and type sudo install-mbr /dev/hda , this will replace the mbr
with a generic Windows version. You can then use the partitioning tools
to either format the Linux partition to Windows format or increase the
size of the Windows partition to fill the hard drive.