OK...that can be VERY difficult to reset, due to the nature of Apple's security measures. And, it depends on if the screen shows a padlock with a firmware password, or if you can get to the login screen and simply forget the password.
-This is the iMac, correct?
-It won't let you insert the OS X Install disc without the password?
-This is running OS X Snow Leopard 10.6.x, correct?
-Does it show a padlock on the screen with a place for a password?
-If you can startup from the install disc, you can follow the steps here:
Reset your lost Mac OS X password in Snow Leopard
Let us know,
C