I recommend
Comodo firewall which is free.
Use Anti-Spyware software.
Spybot is a good free Anti-Spyware program.
Update your security software, OS and scan your computer weekly for viruses, spyware, and adware.
Download and buy software and files from trusted sites you know are virus free like
download.com or
sourceforge.net .
Read the reviews before you download something to see if the software is any good or is bad.