Why there are free antivirus and many expencive ones..which one to be trusted.?

Most, if not all, the Anti-Virus programs started off as free programs, generally they do this to raise the profile of thier product. For a majority of virii you will come across, a free AntiVirus program like AVG will do the job. The free AV [AntiVirus] Programs also tend to be light on your system, whereas the more expensive AV programs are designed for faster computer systems. AVG is a really great AV proggie to get because they have established a reputation and are likely to work a little harder on thier free versions. So for basic AV protection, don't pay a cent because you really aren't going to need it.

I have tried several free anti-virus and spyware programs and I have to say that I found a few that have been pretty good and have helped protect my computer and got the job done well to say the least. For spyware I use, spybot search and destroy and for anti-virus I tend to use AVG free edition. I know that avg has a paid version with some extra features but the free version is enough to protect and get rid of viruses, trojans and any other similar pests. For a firewall, I would use ZoneAlarm, I believe they still have a free version of their software. Also, reading reviews and as you have done here which is get the opinion of others is a really great idea. If you have any other questions please do ask me. Hope you find the program that works well for you. =)

i think spybot is free, it used to be and will pick up things norton doesn't. ccleaner is also good but not virus protection it'll make your pc run faster, when mine slows down a bit i run it.