Hello teekash and welcome to Seven Forums.
Please forgive these questions - I mean no disrespect. Why are you downgrading to XP? Did you know that Windows 7 Pro has an XP Mode? Did the HP 500B MT originally come with Windows 7 or XP? If it came with 7 you may have issues trying to find XP drivers. Did you know that XP is now Microsoft's biggest security headache?
Windows XP Mode - Install and Setup
Windows XP Security Headache
As to why things are taking so long, one more question. Does the HP Restore Plus DVD have Windows 7 on it or XP? If it has Windows 7, why not do a clean install of XP using an XP installation disk? It will format the hard drive (erase everything) during the install process. But you may still have driver issues if the HP originally came wiht Windows 7.
How To Clean Install Windows XP (Part 1 of 4)