I have this computer with the Administrator and Guest accounts. Everytime I try to add a new user to this machine I get a message saying something like "Can't add user: trust relationship error between workstation and main domain". If I try to add a new user using the server's domain instead the message keeps being the same. What should I do? Thanks.

go into MMC and users and computers. Locate the computer account. right mouse click and choose reset account. this should clear the workstation error concering the trust between wkst and server. Give a person a fish, they eat for a day. Suggest they internet search and they learn a skill for a lifetime.

Thanks WANDERER, I reset the computer account as you suggested. Now the message turned to be "Can't grant access to \servername\username because \servername\username doesn't exist" though I already created this user using the server's Active Directory Users and Computers window. What's next?