Can’t dual boot Windows 7 after installing Windows XP

Hi.
I'm kind of new operating systems. Here's my situation. I want to have 2 operating systems. I currently have Windows 7 installed in drive C: when I bought my PC. Then afterwards I installed Windows XP on drive D: but every time I start my PC, it only opens Windows XP with no option to select Win 7.
Why is this so? Any help on this?

