The typical issue is that the Windows installer sees "Linux stuff" in the MBR and decides the drive is no good. One way to proceed is:
- Make a boot floppy or maybe a boot USB drive so that you can get back into Linux.
- Erase the MBR using something like dd
- Install Windows
- Boot into Linux using the floppy
- Re-install GRUB
Almost as easy:
- Use a partition manager to delete all the partitions (or just erase the MBR with dd)
- Install Windows
- Install Linux
Whatever you do, consider setting up relatively small partitions for the OSes, and then one larger partition for shared data.