Well the best defense against infection of any kind is common sense. Other than that, a decent antivirus program would be your best option. I use Norton because I get it free with Comcast. I hear Avira is a good free one. So is AVG. You could try a firewall like Comodo.
Performance wise, just do a defrag every once in a while. Download CCleaner from the Piriform website and run that to de-gunk your computer. You can also edit what runs on startup using CCleaner for a faster boot. There is also a registry cleaner included that I have never had problems with.
Open task manager and kill some processes you're not using. But be careful which ones you use.
etc etc..