I did a quick search and wasn't able to find a definative answer, but I find it very hard to believe achievements are stored locally on your PC. If they were they would be absolutely hackable. I'm sure they are stored in the Steam Community databases.
It would still be smart to backup your Steam folder to save you redownloading, losing save games, resetting your key binds, etc. If you can't back up the whole directory, I'd focus on the *.gcf files in Steam\steamapps. These are the archive files your games are installed from, if you have them it will save you most or all downloading.
Configuration files are in different locations for Valve and non-Valve games. For Valve games look for directories in Steam\steamapps\[yourSteamId]\[Game Name Long]\[gamenameshort]\cfg (example: Steam\steamapps\joe@fake.com\team fortress 2\tf\cfg. For non-Valve games look for the directories Steam\steamapps\common\[Game Name].
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