It's possible to clear the Administrator password of a Windows VPS through the rescue mode which Tilaa offers. Please take a look at https://support.tilaa.com/entries/23767322 on how to start your VPS in rescue mode.
To reset your Windows password, first mount the system partition:
Then use the chntpw tool to change your password:
Unfortunately changing the Administrator password with chntpw doesn't work, but clearing it does. So choose option 1:
Your Windows password has now been cleared. Restart the server using the reboot command.