Have you set up an admin and a user account on the machine?
It can be handy for this sort of thing to have an account you can log into that is relatively clean and untouched that you only use for doing software updates and admin stuff, and a user account (without Admin privileges) that you can screw around with. When you activate fonts, do it only for the user account, so you've always got somewhere you can go back to.
It's also a good idea from a security standpoint not to be using an Admin account for web-browsing/downloading etc.
Make a new account in Sys Prefs > Accounts called Admin - it should automatically be given admin priviliges, but just check that the box "allow this user to administer the computer" is checked. Use a good password. Make sure it's running and happy. Then demote your existing account by unchecking the same box.