Recommended maintenance tasks?

[Rudi von Staden]

While I used Windows, there were a few tasks that I did on a regular
basis to ensure my system was running optimally. I'd appreciate
getting some comments on what is a good practice under Linux, or a
pointer to a good resource on the subject would suffice. Regular tasks
I used to do:

- update virus definitions (is it really true that you don't need
virus protection under Linux <yet>?)
- update spyware detection definitions (is there such a thing as
Adaware in Linux?)
- run windows update for critical updates (apt-get dist-upgrade
equivalent I guess)
- defragment hard drive (necessary under Linux?)
- backup user files and configurations

Thanks,
Rudi