How to improve your account security

This is the topic which can't be overemphasized. Please read these recommendations and follow them if you haven't already done so. Note also that these recommendations work not only for Runescape, they are applicable to the general security of your computer, accounts, email, etc.
If you are not an owner of the machine and there are vulnerabilities, talk to the owner (parent, friend, sibling?) and suggest to fix these vulnerabilities. Play safe.