Some email providers block ALL email from certain other providers (because one system regards the other system as a prolific source of spam). I had to try several different email providers before I could send mail to someone in the USA (because his US provider seemed to regard all non-US servers are potential sources of problems). When I finally found a service that would let me send mail to him I then found that my new email service blocked everything he was trying to send to me!
More specifically, some IP addresses get blacklisted. Your ISP probably uses dynamic IP addresses, so if you switch off your router (or suffer a momentary power cut), and then reconnect, you're given a new IP address. If the one you're given has previously been assigned to a spammer it will listed across the world as a potential source of spam and all mail from it is blocked (even before it gets anywhere near to the spam filters of the intended recipient).
If you're able to see your friend's emails on Virgin Media's website (in a 'Spam' or 'Junk' folder) there will be a button marked 'Not spam' (or similar). Clicking on it tells Virgin Media's server that future mail from your friend's email address (and/or from his IP address) should no longer be blocked, so that you'll be able to see it via Windows Live Mail. (He'll also need to do the same thing to sort the problem out with regard to what you send to him).
If you continue experience problems try changing your IP address (by switching off your router for a short while and then turning it back on again).
Email filtering systems are getting better but they're still not perfect (and never will be). At one time around 30% of genuine mail arriving for me was getting automatically 'junked'. These days it's only around 1% but I still need to keep an eye on my 'Junk' folder, just in case!