I have not done this but if you look at the link I provided on page 111 - these are the steps you need to do to move from a Windows 2003 or in your case Windows XP environment to Windows2008/2012 - you would have to do the same steps - Install vCenter 5.5 on supported hardware and then follow the steps to move the environment to the new server -
The other option is you have a simple environment is to install a clean vCenter 5.5 and recreate your environment -