Hello Ako, welcome to the Seven Forums.
If Virtual PC integration features
are installed and enabled, you must have a password to logon to XP Mode. When integration features are not installed, or when they are installed but not enabled, you can logon using a user account with no password.
If you have removed the password and can't logon, you have to shut down XP Mode, disable integration features, reboot and log in without password, create a new password and re-enable integration features. To enable or disable integration features, go to XP Mode settings.
To setup XP Mode auto logon using any user account, go to XP Mode settings and first delete the saved credentials.
Boot XP Mode, and you should now see this:
Just choose the account you want to use for auto logon, and choose Remember my credentials
More about XP mode user accounts: