I think AVG Free is your best bet for antivirus (http://free.avg.com/us-en/homepage).
What are your specs? Specifically the GPU.
Many people like MSI Afterburner for GPU-monitoring and -OCing.
CPU overclocking you should do through the BIOS.
3DMark 11 is the most common GPU benchmarker; I believe it has some CPU aspects as well.
I use Mozilla Thunderbird for mail, but you may already have a favorite.
I don't know much about defrag software.