All User Accounts

.
Do I have to make a regular scans for spywares/anti-virus in all users account, de-fragrment, clean registry, optimized the registry, etc in all of the users accounts in my pc? What are the things that affects all users even if one user does that things?

 

usually u do because each account has their own folders that may not be scanned from another account. i think spybot s&d is an example of utility thats needs to be run from all accounts.