Buy a new computer.
Seriously - there is no upgrade path. You have to do a complete fresh install of Win7, which means re-installing all of the software that you have under XP pro. So you either get a new computer with 7, so that you can use the XP rig while you set up the new machine, or save all your data, wipe your system drive, and install Win7.
One less destructive alternative is to install 7 in a new partition on blank space on your system drive, if you have that much free. It's an excellent approach, but a bit risky for a newbie (if you are a newbie). Do complete backups first, so that you can recover from a disaster.