Could you post back with a copy of your register and login page. I will take a look and let you know what I think.
One problem that occurs when using encryption for the password is that on the login page you need to also encrypt the value that the user is entering for the password so that the compare works out correctly.
Another issue that I have seen is that the password column is not defined as a varchar of large enough size. It should have a size of at least 40 to allow for the full length of the sha1 hash.