When you install and launch Windows XP Mode for the first time it has following account related defaults:
- Default user: XPMUser
- Integration Features enabled
- Built-In Administrator account disabled
- All user accounts must use / have password
When Integration Features are enabled on any Microsoft Virtual PC virtual machine you can only use and login to accounts with a password. Only way to use accounts with no password is to disable Integration Features. This is not recommendable as so much of Virtual PC virtual machine's functions need integration to work properly.
To be able to run programs as Administrator or login to Administrator account you need to enable the account and set a password for it. This tutorial will show you how.
Notice that this method can also be used on all other Windows XP, Vista and 7 virtual machines running on Microsoft Virtual PC.