There are no known bad anythings for OSX. There has never been a report of an OSX machine getting anything nasty that I'm aware of. Trust me, if anything does pop up, the high tech press will be all over it.
AS you've probably found if you log into the machine, everytime you want to install any sort of executable, osx hits you with the admin password requirement.
To be doubly safe, you can create a new non admin user account and use that 99% of the time.
Using OSX 10.3.9 on a G4