I use UBUNTU and Windows XP in dual boot on my comp…Recently, I reinstalled Windows XP….which as expected installed windows loader over grub…which doesnt detect linux partitions…so heres what u can do to recover grub and there is no need to install ubuntu again 
….
1. Boot the Desktop/Live CD.
2. Open a terminal (Applications -> Accessories -> Terminal)
3. Start grub as root with the following command :
sudo grub
4. You will get a grub prompt (see below) which we will use to find the root partition and install grub to the MBR (hd0,0)
[ Minimal BASH-like line editing is supported. For the first word, TAB lists possible command completions. Anywhere else TAB lists the possible completions of a device/filename. ]
grub>
Type the following and press enter:
find /boot/grub/stage1
Using this information, set the root device:
grub> root (hd0,1)
Install Grub:
grub> setup (hd0)
Exit Grub:
grub> quit
5. Reboot (to hard drive). Grub should be installed and both Ubuntu and Windows should have been automatically detected.
More details and options avialable : here
