When I open a .docx file directly from a RAR archive using WinRAR, where does WinRAR extract the file? My OS is Vista. RAR version 3.9.
Winrar extracts the archives to
C:\Documents and Settings\<user_name>\Local Settings\Temp (Windows XP) C:\Users\<user_name>\AppData\Local\Temp (Windows Vista/7)
by default, but I don't recommend you to edit an extracted file in a temp folder. Instead, extract the file (for example, to desktop), edit and save it, and then update the archive with the new file.
P.S.: You can change Winrar's default temp directory from Winrar's Options > Settings > Paths tab.