A Digital Signature certificate (DSC) is kept in internet explorer of computer system (PC) but keeping DSC on your computer system has following draw backs :-
a) It can be misused by anyone who is having access to your computer system.
b) DSC is lost if computer system is formatted or internet explorer is changed.
Accordingly, safe and proper method is to keep DSC on e-token, a small USB port devise, which is password protected. The said e-token is a small hardware device and can be plugged to USB port of any system to digitally sign the documents and when not in use can be kept in safe custody.