>On SQL Server, I got this error this morning:
>
>"Transaction (Process ID 101) was deadlocked on lock resource with another process and has been chosen as the deadlock victim."
>
>What is causing this and what can we do to assure that this will not happen?

There's no simple answer to your question. You can start by reviewing following topics in the BOL:
- Deadlocking
- Minimizing Deadlocks
- Analyzing Deadlocks with SQL Server Profiler