Hello Parallels support,
I'm not the only person with this problem. I think it needs more attention by you. See this forum thread:
In my case and others' experience, the installation of P7 led to an automatic login account in the Windows VM. Because I was not prompted to create an account, I did not record a password. Indeed, I never knew a password and I think the password was actually blank or empty. When Windows later asked me to change the password, I had no idea what the original was. I tried some usual passwords and none worked. Leaving it empty is certainly not usual for me, but I happened to try that and it worked. That's how I got out of the jam and was able to set the password to something that made more sense. I was just lucky.
I think you need to change the Parallels setup system so this kind of thing does not occur.