Is it safe and reliable to backup MySQL databases by backing up the data folder? I have a MySQL database [server version 5.0.x, type InnoDB] which I want to backup. I don't have username/password to back it up using mysqldump. My question, is it safe to back up the database by backing up the entire MySQL's data folder (of course, after stopping the MySQL server service in Windows 7)?
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!