To have the files safe , you've got to make the iMac password protected as well as to have it to boot-up upon authorization . This makes no body to have a backdoor access into your iMac through floppy drive or a CD drive .
You may want to take a look at the machine's Setup menu for the security settings including User Password . Then , when it prompts , just give a password of your choice ; there is a word of caution here - that resetting boot-up password is a brainer by itself so chose and type but carefully .
Again please note that some machines will allow you to add the Setup password so to contain any unauthorized access to Setup program too , check the motherboard manual to doubly make sure if the feature is available on your machine .
Majority of system motherboards have switch or a jumper for resetting the boot password should the need arise . In that case switch off the system and off the plug , open up the system case , and just reposition the jumper ; take care that you ground before doing this job .
However , in case the jumper is not there set up of the password , you may have to look for that switch that will reset your Setup program ; implies it will clear boot-up password and erase other settings as well .
All these efforts are for one main objective to keep the system safe ; your iMac off the
wrong hands . Despite these to have the boot-up password disabled is not a big deal and it isn't that difficult to take off the hard disk , frisk all the files and folders and replace with no signs of tampering too . So how to keep the intruders off your expensive iMac ? Locks that come in the $30 range will be effective for simple systems and not an iMac . Lock that is made for iMac will serve the purpose ; G5 custom locks or an Apple MacPro locks do . There are advanced physical security devices including iMac plate lock devices that are barely visible yet offering very good computer security .
The iMac locks may not be cheap computer security devices , but instead of having to bear the loss of revenue by computer vandalism and to avoid the the company's reputation being ruined and investing on administrative expensive for repairs post vandals attack , it makes sense to invest in the iMac locks and be secured . There are some products that offer iMac insurance too with the iMac lock purchase , just do your diligence and you will be at peace .