ASP.Net 2 And MS SQL 2005 SP1 Transactions?

[buzzzz1]

Hi,
I have an issue where transaction deadlocks are created from time to time.
But unless my understanding of deadlocks is fundamentally floored then these deadlocks should not occur.

The code in one web site creates a transaction and starts performing several database statements.

The code in another web site is creates and database connection and a command object then runs a complex select statement (normally takes around 20 seconds).

How can a deadlock occurs when one of the 1 of the 2 conflicting statements contains no transaction and is only a select statement?

I could understand a normal lock, as the select statement may need to wait for the transaction in the other site to end but not a deadlock.