unable to install xp to existing vista system

I don't have the Vista disc (the computer came with it already installed) but I do have my XP disc. I have partitioned the hard disc and have ample space to add another OS but when I put in the XP disc the "install" option is not available. The other two options = perform additional tasks and check system compatibility are open, but those don't seem to help either. I also can't "check system automatically", but it allows 'visit compatibility web site'. The laptop I'm trying to add this to is not connected to the internet (I have only dialup on my desktop). Is there something wrong with my disc? Have I done something incorrect on the laptop?
I tried the Windows website, but it sent me in circles.
Thanks in advance.

I managed to get the 'install' button to activate, but now I have a popup that says "This program is blocked due to compatibility issues". Setup cannot continue because the version of Windows on your computer is newer than the version on the CD.
The tutorial I was reading didn't address this issue either.
Thanks again.