Individual startup folders are located at C:\Users\$USERNAME\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup .
The shared startup folder is at C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup; any shortcut placed here will spawn an application on any (normal) user login.
(The “contents” of a user’s Startup folder is the union of these two folders. If memory serves, the user folder will override the system folder for items with exactly the same name. Note that executables can be dropped into this folder in addition to shortcuts, though doing so may make it more difficult to hide a backdoor…)