Why are you going from Windows 7 to XP? That's like going from XP back to Windows 98.
The hidden partition you're trying to delete is probably the Recovery Partition which you can access by pressing a Fn key during boot (usually F10 or F11). If it's not or you can't get to it, as long as you have a valid Product Key on the COPA sticker attached to your computer, you can download a legal copy with SP1 integrated from here: Official Windows 7 SP1 ISO from Digital River
Make sure you get the same version you have, Home Premium, Pro or Ultimate and 32 or 64 bit. Note that Basic or Starter is not available.
I recommend using ImgBurn
at 4X speed (or the slowest available) to create the DVD from the downloaded .iso file.
You can do this on any computer capable of burning a DVD.
For techies or folks who work on computers: Create Windows 7 Universal ISO With All Editions Selection On Install with ei.cfg Removal Utility
This will fit on a 6GB flash drive or can be burned to a DVD.
Good instructions here: Clean Install Windows 7