There are many reasons to reset your Windows XP Mode. When you forger your password and If you absolutely don't remember credentials or When your windows XP mode has been affected with virus and malware. Then the only option left is to backup data from VHD (if it is important) and reset the XP VHD.
To again reinstall XP VM do following
- Take backup of data inside VHD by mounting it as disk (use diskmgmt.msc)
- Make sure VPC.exe is not running
- Delete diff disk (you can get the path by using settings), delete .VMC, delete .VMCX, delete %localappdata%\Microsoft Virtual PC\options.xml
- Click on Virtual Windows XP link, this should set XP VM again