system
15
Believe me, you have not deleted all remnants of passed software, Macafee & Norton are or were the biggest players to leave crap on your pc.
If you want to play safe I suggest this http://www.microsoft.com/en-gb/security_essentials/default.aspx
Some will recommend this http://www.avira.com/en/avira-free-antivirus FREE
Or this http://free.avg.com/us-en/free-antivirus-download FREE
Try staying away from testing too many Antivirus programs, unless you learn to clean up afterwards.
I personally would look at a stronger firewall too.
You can download Malwarebytes free & use for scanning for added protection, but it’s not real time.