MS provides "sp_getapplock", and the corresponding "sp_releaseapplock", to help you do this. With them, you can leverage SQL's already-built-and-tested locking mechanisms for your own logical locking. You also want to look at APPLOCK_TEST() (and maybe APPLOCK_MODE()).
SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.