Problem with Queued updatable subscriptions

Hi All,I have a problem replicating data from Subscriber to Publisher.I configured Queued updatable subscriptions in transactional replication. Publisher is on SQL Server Standard edition and Subscriber is on SQL Server Express edition, both on Windows XP. When I insert/update data in publisher, the data is replicated to subscriber. If I insert/update data on subscriber the data is not replicated. When I see the replication monitor for errors, it shows an error "Cannot load a DLL xprepl.dll or one of its resources".Could any one give me a solution or please tell me if I missing something?Thanks you all,Ravi.