Significant Performance Difference If SELECT Command Contains User

SQL 2000 Connection String:
user id=MyUserName;password=MyPassword;initial catalog=MyDB;server=MyServer;Connect Timeout=30 
This SELECT statement returns its 10 results nearly instantly:
SELECT * FROM MyTable
Ditto from above, but completes in 30-40 seconds:
SELECT * FROM [dbo].[MyTable]

Ditto from above, but completes nearly instantly:

SELECT TOP 1000 * FROM [dbo].[MyTable]
Obviously I have stopped using the [dbo] syntax in my SqlCommand's (SELECT's and EXECUTE's) but still would like to know why this is.
vr, Rich