Ok, it sounds like your server i hating you
Firstly, see if you can log in as the Administrator user, which you would have setup during the xp install, that should always be allowed via remote desktop (i thinK)
then try this:
1. Click go into computer management, through control panel>Admionistrative tools>Computer management
2. In the left tab double click local users and groups
3. Then double click on groups
4. Then in the right pane double click on "Remote Desktop Users"
5. Finally check that the users that you want to be allowed to logon are there, if they aren't click add and go through the process to add them.
The users that yopu should add are the users on the server, not the user for your computer you are trying to connect to it with
Good Luck
Lee