There's a lot of options around MSMQ and WCF. There are a couple of different methods and frameworks that could help, depending on many factors.
If you would like to do it you self with a little guidance, i suggest that you take a look at Tom Hollanders great posts:
There are other more "complete" options if you're into using an existing servicebus such as BizTalk, nServiceBus or MassTransit.
Another option is Multicasting wich is available since MSMQ 3.0, which is such a feature that will support scenarios where you have one publisher (your windows service) to multiple recievers (your windowsapp and webapp). Both sender and recievers can use the MsMqIntegration binding to interact with the queues.
I suggest that you take a look at: http://blogs.msdn.com/b/solutions/archive/2005/09/20/471615.aspx