If you have two users on a Mac that both want to use Parallels it appears that you have to install Windows twice. Can this be the same serial number ?
I have followed those instructions to the letter, but still have a problem. Works for the first user, but the second gets the error - "WINXP.HDD the file is missing, corrupted or used by other appplication" I have definitely closed down the other instance of Parallels, and the permissions are read/write to every one. Tony