This is a very old thread. Please do some google work or at least search through posts in this forum, before posting it.
To restore Windows XP MBR
1: Boot with Windows XP CD in the drive
2: Hit any key to boot from the CD
3: Wait for installer to load drivers in memory
4: When asked to press enter to install, type: r
5: When asked which Windows installation would you like to log onto,
if there is only one, type: 1 (and hit enter)
6: When asked to type the Administrator password, I hit enter without
typing anything which will be the administrator password for most OEM
Windows installations, (No password). If you installed Windows XP
yourself and added an administrator password, type this password.
7: At the prompt type: fixmbr (and hit enter)
-**CAUTION** and a warning appear
8: When asked "Are you sure you want to write a new MBR?:" type: y
-The new master boot record has been successfully written.
9: When the prompt returns, type: exit
-The computer re-boots and Windows kernel is loaded.
After that, you can again install grub through rescue mode. After installing, you will get a dual boot.
This will work fine if you have not corrupted your windows partitions while resizing Unix file systems.