an operating system is mandatory. joke
i think it is very important to have a good firewall, virus guard (I use kaspersky internet security), and anti-spyware protection.
and, a software to clean the registry, check shortcut errors, etc, I use tuneup utilities. there could be several, they help to tweak the os, and also keep the os in tuned.
a disk defragment software. I use diskeeper. if you do not defrag, you'r system performance would be significantly decreased. diskeeper has a frag-shield it will automatically defrag ur volumes in the time data is being written.