New server registration error

Hi, I've just installed MSDE 2000 Release A on a Windows NT Server 4.0 w/SP6. I'm on a WinXP machine and I've installed the Enterprise Manager, and am trying to register a new server registration. However, I'm getting the error: Login failed for user 'sa'. Reason: Not associated with a trusted SQL Server connection. I've made sure that both the client and server are using TCP/IP for the protocol list, and I've tried using the physical server's name and it's IP address to register, but keep getting the same error. Thanks for the help.How are you authentacating? Try a search before posting.
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http://search.msn.com/pass/results.asp?RS=CHECKED&FORM=MSNH&v=1&&q=Login+failed+for+user+%27sa%27.+&cp=1252|||I was trying to authenticate via SQL Authentication. I did try searching, and nothing I found seemed to pertain to my situation. But I fixed the problem, so if anyone wants to know, I used Windows authentication to register the server, then I right-clicked on the server, chose properties, and went to the security tab. In there, it was checked to use "Windows only" under the Authentication section. I changed it to "SQL Server and Windows", restarted the SQL service, deleted my Windows SQL registration. Then I was able to successfully register my SQL server using SQL Authentication!