The user name and password it is asking for is the same user name and password you would use to log into the computer you are trying to access. The reason you are not able to change said user name is because there is only one authorized user on that computer: the account you created when you first booted it up.
Right-click on My Computer, click Manage, click Local Users and Groups, find your name, right click on it, click Set Password. Make sure you decrypt andy encrypted files on your computer first. The other way to do it is though the Control Panel, but you don't feel quite as accomplished. My suggestion (and Microsoft's "least administrative effort" best practive policy) is that you make both computer's login user names and passwords identical. What this does is, when Computer A asks Computer B for files, and Computer B asks Computer A for the right username/password, Computer A already has the right credentials (since that's what you used to log in). Thus, Computer A will never ask you for a password again.
Hope this all helps.