If it's not the folder where Powder Toy was extracted to, and you can't find it by searching for powder.pref, then I have no idea.
Maybe try, in the TPT console, running fs.makeDirectory("tptasdf1234") (or some other unique name). Then search your entire computer for a folder with that name.
Your powder toy directory is where you have powder toy, if you have The Powder Toy(the one that you open to play) in your desktop, then your desktop is the powder toy directory.
I eventually managed to find a copy of Vista, and for me the TPT folder seems to be:
(TPT should probably store data in a consistent place, such as %APPDATA%\PowderToy\, instead of in whatever mad location Windows decides the working directory should be...)
Read all posts not just the last ones as this threat is still small...