[ubuntu] Have "lost" Ubuntu boot-up menu!

Hello,

I had a dual-boot Ubuntu 12.04 / Windows XP system. XP became corrupted so I decided to install it again.

Now, when I boot-up the machine, I no longer get the Ubuntu menu. I can only login to XP.

I did not delete the Ubuntu partition so 12.04 is still there.

How can I get the Ubuntu menu back on start-up so that I can select whether I want to load Ubuntu or XP?

Thanks.