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If you used an HP CD to revert to factory settings, put it in again and see if it includes a repair option. If it's a Windows XP CD it will definitely have that facility. Either way, if you can get to a Command Prompt - try tapping Function 8 as you power up the machine and see if you get the Advanced Boot menu - try typing this at the C:\ prompt
expand bootmgr temp then press Enter and at the next C:\ prompt, type
attrib bootmgr -s -r -h and press Enter again,. Restart and post back what happened next.