You don't. Achievements are kept on a Valve database called the steam cloud. It is 99% impossible to un-achieve achievements.
Yes.
Sorry for such a short answer. No, you may not.
Its another Achievement but to complete it you have to finish 5 or more normal achievements.
Delete local files and redownload the game.
no
none specifically. Team Fortress 2 does NOT give you a virus.
Team Fortress 2 was initially a Quake Mod (Team Fortress), for Capture the Flag and Team Deathmatch
Team Fortress 2 is the sequel to the video game Team Fortress. There are similar characters and missions in both games, but the main differences are the new maps and new statistics capability in Team Fortress 2.
Team Fortress Classic and Team Fortress 2 is developed by Valve Inc.
No, there are no throwable grenades in Team Fortress 2
Team Fortress does not have levels.
You can find Team Fortress 2 at store.steampowered.com