let me get this straight. You have a serial number to go online but you can't "log in"? Have you already used the serial to set up an account once? If so, then you need to use the "Old Profile" button to retrieve your old profile and fill out the data ( name, etc...) you have used the first time when you used your serial.
If however, you don't remember what name, email adress you used for this, then perhaps the server administrator for Cossacks 2, L'Europeen might be able to help you with this to "reset" your account.
I guess you can send him a private message here on the site, he often visits. See to his profile for a private message: http://cossacksworld.ucoz.co.uk/index/8-214
If he doesn't answer, I'll attract his attention to this thread.
Hope that helps.