When installing Ubuntu you may delete MBR. MBR point grub instead of Windows bootloader. If you want to move Windows bootloader again you can use fixmbr command.
Fixmbr - Repair Master Boot Record with MBRFix from Windows 2000, XP etc. instead of using fdisk /mbr. FixMbr could help you recreating original master boot record (MBR) that works with any Windows (Win2k), XP, 95, 98 when Linux LILO damaged it.
download from here http://www.ambience.sk/fdisk-master-boot-record-windows-linux-lilo-fixmbr.php
If you have one disk enter this command. If you have more than 1 physicial hard disk (hard drive), you have to use proper number for your disk
mbrfix /drive 0 fixmbr
If you could not boot your OS then use Windows Cd boot in recovery mode and in the command prompt use fdisk /mbr