In Windows XP…
"C:\Program Files\Microsoft Office\Office\MSACCESS.EXE" "YourDriveLetter:\PathToYourDatabase\YourDatabase.mdb or accdb" /decompile
In Windows 7 64 bit…
"C:\Program Files (x86)\Microsoft Office\YourOfficeFolder\MSACCESS.EXE" "YourDriveLetter:\PathToYourDatabase\YourDatabase.mdb or accdb" /decompile