avast very lite and good for me, i'm use it

Eset, light and effective

ESET nod32 does it all

I use Nod32 for years, never doubt about it!

Eset light and powerful

Well I do not use any anti virus protection. I have 4 computers on 95% of the time and use only malwarebytes and wise care 365. I backup all my computer with acronis true image and in the last five years maybe used it twice (always had recent backups). I find that they cause more problems then they help. Especially with programs that uses key generators and find other so called "bad" infected files.
I also use sparingly wise Care365 for PC scan and that is across windows XP and Windows 7 and no problems.
Malwarebytes is the best!

i do not use an antivirus.

Kaspersky, but if you're looking for some free alternative it's Avira

Go for bitdefender antivirus. it is very good.

And not all Anti-Viruses slow down your computer too much... If they use on access scanning then yes, they are heavy. But there AVs like Comodo or Norton which only scan the files when they change. On access scanning means it scans every time you open the file so of course it slows down your machine...