Do you know which drive's MBR grub is installed into now? It is better to have the Windows boot loader on the Windows drive and grub on the Linux drive.
If you do a new install the old grub will not match and not work.
You can install grub2's boot loader to the Linux drive with this, Then change BIOS to boot from Ubuntu drive:
reinstall from working (not liveCD/DVD/USB) system - first find Ubuntu drive (example is drive sdb but use your drive not partitions):
sudo fdisk -l
if it's "/dev/sdb" then just run:
sudo grub-install /dev/sdb
If that returns any errors run:
sudo grub-install --recheck /dev/sdb
You can also install a Windows boot loader to the Windows drive if you have grub in it.
How to restore the Ubuntu/XP/Vista/7 bootloader
You can also manually install just the lilo boot loader to the MBR as it works just like the Windows boot loader.
Restore basic windows boot loader - universe enabled if error on lilo not found
Simply open Synaptic and Settings > Repositories and tick the box against the Universe repo in the Ubuntu Software tab. Close that window and click on reload before installing lilo with Synaptic or command line.
sudo apt-get install lilo
sudo lilo -M /dev/sda mbr
May show error messages about the rest of lilo missing, ignore, we just want MBR with bootloader to boot partition with boot flag (Windows).
If you have grub in sdb, then change BIOS to boot from that drive.
When you reinstall a LInux system, use Something else and on manual partitioning screen choose the Linux drive from combo box at bottom of that screen to make sure grub is in the Linux drive.