The method for accomplishing this in *Nix may vary from vendor to vendor, but commands such as "chgrp" (change the group ownership of a file), "chmod" (change the permissions mode of a file) and "chown" (change the owner of a file or directory) can be used to modify things the way you want them.
Another trick you can use is to rename the Administrator account. Again, viruses and hackers tend to target the Administrator account or accounts with administrative privileges because they provide the broadest access. If you rename the Administrator account to something else you can make it at least a little bit harder for an attacker to find it. You can follow these step-by-step tutorials for changing the name of the Administrator account from Auburn University:
Because a hacker knows that there should be an Administrator account, you should also create a replacement account called “Administrator” and give it very limited privileges like a regular user. When a hacker comes searching they will find the Administrator account they covet and if they succeed in hacking into it they will not have gained the omnipotent authority over your files and folders they thought they would get.
Admittedly this trick is by no means foolproof. The fact of the matter is that each user account created in Windows has a unique SID (Security ID). Part of the SID is the RID. By default, the Administrator account has a RID of 500 and the Guest account has a RID of 501. Experienced hackers know how to decipher the SID and can tell based on the SID whether the account is the true Administrator or not. There are also script-kiddy hacker tools that will automate this process for less-experienced hackers.
However, that is not a reason in and of itself not to do it. Locking your car door won’t keep professional auto thieves from taking it, but it will keep the general population from taking stuff from your car. The same logic applies here. You won’t trick everybody or keep everyone out of the Administrator account, but the more difficult you make it for the general population the better off you are.