you can take a back up of the MBR and install the windows.
After installing the windows, you will find only windows boot loader. After starting the windows, restore the old boot sector.
You can download the tool from here
http://www.diydatarecovery.nl/mbrtool.htm
you can take the backup in your hard disk itself if you have a seperate partition other than windows installation partition, you can simply copy back that MBR.