Boot to Linux, and open YAST. In there, go to "System", then "Boot Loader". In there, open the tab "Boot Loader Installation" and click "Other". From the menu select "Restore MBR of Hard Disk".
This will give you your original MBR back and you can easy boot Windows safe mode. After you are done with your Windows configurations, boot the PC from the Linux CD and you will find some menu like "Boot Loader setup" or similar. Here you can re-install Grub and everything will be fine
Worked for me that way in SuSE 10. A little hassle but far less than reinstalling any of the whole OS's.