Team Fortress 2 Servers not Showing on List

I ran into an issue a couple days ago and it was when I was playing Team Fortress 2 by Valve Software. When I called up the Server list, the result was an empty list (no servers responding to the query). I then backed down to the Steam Client and tried there with the same result (but only with the TF2 servers; all other game servers appeared fine). I eventually was able to get the Steam client to show TF2 servers by deleting a file from the TF2 folder (I forget which one off the top of my head, but I found it in the Support section of Valve).

When I entered TF2 and tried again, I still didn't have a server list. Long story short, it was my Avast A/V that was causing the issue. The nice thing was updating the A/V (definitions as well as program) resolved the issue.

My thought is a file was put into the A/V definitions on one update, then removed on the next. Needless to say, I'm able to play again. Woo-hoo!