This is a destructive process so back up your files. Afterward you'll need to reinstall XP if you're not ready to migrate fully to Win7. Try first installing your older XP programs in Win7 http://www.sevenforums.com/tutorials/316-compatibility-mode.html or if necessary run them in XP Mode in Ultimate, or in a lesser version use free Virtual Box or Virtual Player.
You could also install Win7 to another HD to Dual Boot via BIOS.
To access the Win7 DVD Command Line, boot the DVD, press Shift +F10 at first screen, a Command Box should open. If not, select Repair My Computer on second screen, click through to Recovery Tools list to choose Command Line.
Have a look at Method One in this tutorial at the link below for an outline on how to get to DISKPART so you can use the "Change a Dynamic ... " tutorial listed above and be sure to post back with any further questions you may have and to keep us informed.