C:\Documents and Settings\[user]\My Documents\Bus Driver
Vista / Windows 7
C:\Users\[user]\My Documents\Bus Driver
The game saves are under the folder /save/
There is a save file for each bus route that contains the high score lists. I you played some bus routes on one computer and played others on another computer, you could copy over the route files to have all your plays together.
Download the save files to unlock all routes: BusDriverUnlockedSaveFiles.zip