- Buy another hard drive.
- Install the new drive as a second drive in your computer.
- When the windows installer asks you where to install Windows 7, tell it you want to use that second drive. This will often be drive D or E, but the size of the drive should be apparent in the installer.
- After the install, your BIOS has the option of booting from any drive you like. Typically you press F8 when your computer is in the Power On Self Test screen, but it all depends on your computer's BIOS.
- If you're happy with Windows 7, run the Settings Transfer Wizard under XP then under Windows 7. Copy your data to the new drive and then erase the old drive. You can then either remove the old drive or re-install it as a secondary drive for data.
This is certainly the safest method for doing what you want, and it's probably also the easiest. Mucking around with your hard drive's partition has been known to destroy data, especially if you don't do it right. Moreover, re-partitioning the drive after you decide that you like Windows 7 (or not) is awkward at best and equally likely to destroy your data.