I did this too not to long ago while updating to Win7.
Had a rar file with all my game data, music, and personal files and a second rar with my sdkcontent backup-ed. Forgot to merge them and lost everything.
Thought all was lost then I started researching ways to get it back and found the EASEUS recovery tool. Wasn't perfect as most of the files were overwritten though.
Whats worse is when I opened the .vmf with the recovery tool I could see the chunks that had overwritten content on them, like a crappy game I installed, went over the vmf which (the parts that I could read) showed the install directory of the game. Still found/recovered 70%, much better than 100% I'll say.