Ignore roufroy. He doesn't know s**t about this (sorry but its true). I had this problem myself, but I haven't been able to work out the cause. I've personally reinstalled steam, TF2, and source
SDK/source SDK base/base 2007/refreshed sdk content/checked cache/etc, none of it helped. When did it start? Did it work the first time, then stop working? Did it just not work all along? What operating ...