Grub is usually installed to MBR (Master Boot Record) it should get overwritten during windows installation.
If not you could boot the windows cd and choose shell and try to give it command fixmbr. It should write the ntloader to MBR.
If you still get working grub shell you can load windows from there by something like this:
chainloader(hd0,0)+1 [enter] boot [enter]
Hope it helps.