I REQUIRE ASSISTANCE!

My TF2 keeps crashing everytime I join an ONLINE server. I defragmented TF2 and it still doesn't work, if anyone has any suggestions, please respond!

First thing, this should be posted in the PC Workshop section, as where all PC problems on this forum should be posted.

This problem happens to me all the time on TF2, Left 4 Dead and Left 4 Dead 2. It's a common problem, the Steam forums are full of threads on this, and yet nobody got a clue to why it's happening or got a fix for it.

Sometimes, I can play 2-3 hours of TF2, then go to play L4D and it will disconnect about 30-60 seconds after joining a server, each and every time. Then there are other times, where I can play a full night of L4D, but when I go over to TF2, I will get disconnected from every server I join.

It's one of 3 things, steam (or steam cloud), anti-virus program, or another program I have running at the same time. Sometimes, I even get off mumble to see if that works.

Sometimes it does and sometimes it doesn't.