Just done the same thing myself.
Got XP (in sda1), Vista(in sda2) and 9 Linux. Ubuntu in my case is in sda6.
No need to reinstall anything. Just boot up Ubuntu CD. I am using my case as an example because sda6 is the 6th partition in the first disk (hd0). It is known to Grub as (hd0,5) as Grub counts from 0. One can ask Grub to "source" the Grub files from (hd0,5) and install itself in (hd0) which is equivalent to the MBR. I has Ubuntu in a single partition. If you have it on multiple partitions it is /boot partition you need to "source" the Grub file from from. Here is the command in Ubuntu terminal ( adjust to suit your case)
The read command are issued inside the Grub shell. Other commands are in Bash terminal.
The above will restore Grub.
If you boot to XP, which is dual booting Vista, then you can select either XP or Vista.