They're not located in a folder.
Apps wishing to install an icon in the system tray make a call to ShellNotifyIcon to put the icon there.
These apps are typically launched at startup, and apps launced at startup can come from a few places:
-The user's Startup Items folder
-The "all users" Startup Items folder
To get to those, right click the start button and select either "Open" or "Open All Users" and go to Programs, then Startup.
Or, they can be launced from the registry at the following location:
HKLM\Software\Microsoft\windows\CurrentVersion\Run
HKCU\Software\Microsoft\Windows\CurrentVersion\Run
These can be found using regedt32 or regedit.
Is that the information you needed?