Hello sachithkn Welcome to the Seven Forums!
One option available would be replacing the Grub loaders entirely and repairing the 7 mbr. Afterwards the free program called EasyBCD
can add a boot entry for ubuntu into the 7 BCD stote(boot configuration) as well as another entry for XP. You would then see the 7 drive made the default boot drive and decide which OS would set as the default.
A good alternate site for that if you would rather not register for the free for home use download is available at Download EasyBCD Community Edition 18.104.22.168 Free - A handy tool for tweaking your system - Softpedia
When installing ubuntu last one tip is seeing the root partition made the "? mount point" avoiding seeing Grub installed into the Windows boot sector. The EasyBCD docs
explain how the program will still add a usable boot entry in for the other OS.