You should take a look in the Books Online help that came with SQL Server 2000. There is a section that deals specifically with deadlock situations. It explains what causes them and gives ideas on how you can trace them, for example, there are trace flags that will tell you which resources and threads are involved in the locks.
I'm sorry to just point you to the help system, but your question is too vague and the possible causes of your problem are too many to try and give more specific advice on.
