My Dell motherboard crashed (XPS 400) and I rebuilt the computer with new hardware, using the old hard drive.

When I reinstall from a CD, I direct the OS to install on the old 4 GB partition that held the backup OS. It's successful, but also installs Program Files folder, etc., and there's not enough room to install more programs. I'd like to install on C:, but it still has the old OS, and I fear a conflict. I'd like to keep the data on C and not reformat.  What do you suggest? Thanks....

A second question is if I reinstall from a Dell reinstall CD onto a new hard drive, will the original XP product key work, that was assigned to the factory installed OS now on the hard drive? I'm wondering if reinstall Dell CD's are assigned a specific product key...