Hello.. I have a lenovo netbook... I don't have any updated antivirus running in it...

It came preinstalled with norton antivirus which had an expiry date... can anyone suggest me with the best antivirus available online for free... I can't install an antivirus in netbook using a cd... and don't want to pay online through credit... or it will be better to confess I don't hold great knowledge about the topic...

I would be thankful if anyone cares to explain me in detail

Best is a relative term, but there are a number of free anti-viruses out there:

avast! Free Antivirus - Download Software for Virus Protection (http://www.avast.com/free-antivirus-download)
AVG Free - Antivirus Free Download | Free Virus Protection (http://free.avg.com/us-en/download-avg-anti-virus-free)
Avira AntiVir Personal - FREE Antivirus (http://www.free-av.com/it/trialpay_download/1/avira_antivir_personal__free_antivirus.html)

There are lots of excellent free antivirus software products available. My personal favorite is Comodo (on a Lenovo laptop) and the most popular probably is AVG. To find other good choices check out this site that I use: freeinternetsecuritysoftware.com where they have all types of security software sorted by category.

The best is a very relative word. That's like asking what Computer Brand is the best. Everyone will have their opinion. It comes down to what you need it for.

I myself use AVG for my home PC's.