Sandboxie is light on resources, even in its default configuration which installs a Windows service, SbieSvc.exe, and a startup program, SbieCtrl.exe.
BTW, Sandboxie does not ask you to "disable all protection" while installing. That's unnecessary.
Edit: On my XP box,
SbieSvc.exe is using 2440 K of memory.
SbieCtrl is using 6224 K.