u can re-install the MBR but in this case u will have the MBR containing the windows boot line only which wont contain the Linux , then if u know how to add the line for the linux booting then u should add it.
If the case you wanna take back ur window$ boot. Try this:
Put ur windows installation CD
Press R for recovery console
fill password (if ur windows have admin's password)
in the console type help (if you wanna know all the command)
if you wanna take back mbr from linux/ recover ur bad mbr (it happaned to me a lot) use fixmbr.
i have a better solution, change the grub for lilo, lilo is better and easier to configure, if you are using a based debian distribution do : apt-get remove grub, apt-get install lilo, adn then vi /etc/lilo.conf, and this file is really easy to configure, just try