Use a boot disk; doesn't even need to be a current one. If you have a Windows 98 boot disk around that will work. When you boot from it, it should load the MS RAMDisk and assign it a drive letter. Into that RAMDisk it should load several utils including fdisk. Run this:
X = the RAMdrive letter
That should clear things out so Windows can reassert control over the MBR and load its info back in without Grub's remnants causing issue.