The vast majority of viruses come in when the user visits dubious web pages with a vulnerable browser, or opens dubious emails. As long as you don't use your living room's audio system for that, you will automatically be safe for the most part...
The rest are worms or other attacks that go through vulnerable services in the operating system or the applications that are running on it. If you have a firewall or NAT enabled in your broadband router or whatever it is that brings the friendly interweb into your home, it is practically impossible to get infections that way (unless you go looking for them, see paragraph 1).
I don't know if the BM5 has the firewall of the OS enabled - probably has - but it will help as well. So does that fact that it is not regular XP, and so it won't be running many of the services that would be enabled in the full version.
If you go mucking about with the OS yourself, installing new applications or other features not supported by B&O, you will of course be on your own regarding how safe the end result will be.