Is it safe and reliable to backup MySQL databases by backing up the data folder?
Take a look here.

In a word, a resounding "yes" is the answer to your question provided the server is stopped!

I'm not sure that there are any hot backups available for Windows - Percona's XtraBackup only works on Unix (AFAIK). It's possible that MySQL have a Windows solution for hot backups, but there's nothing Open Source - (again AFAIK, and it's been a while since I've looked at this issue). 

But, you are perfectly safe with a cold backup!

I'm just wondering how you're planning to make use of the cold backup in the absence of a password? Does the app that uses it have it's own table dumping facility? My advice on this is to figure out how to get your raw data back ASAP!