Cant log in to backoffice when uploaded to webhost

[Tomtefar]

Hi,
Have a new problem, maby someone know how to solve it (have searched the forum but cant find a solution).
I just rented a webhost, after some learning i managed to install the database, upload the shop and get everything to work. Although when i try to log in to the backoffice it says "bad user or password". Im 100% sure its right, works great when i use it on WAMP.
Have tried diffrent browsers, empty cookies etc. I added a new user with the "newadmin.php" found in the forums and with that i can get access and change the password on the existing admin. When that is done everything works great until i upload a new version (im developing everyting localy). Then its the same thing and same solution.

Why does the password to the existing admin account change/stop working when i upload it to my webhost?
Thankful for all help i can get!