http://windows.microsoft.com/en-US/windows/products/security-essentials 

or AVG. 

If you've got Comcast as your cable provider, you get to download Norton or Mcafee for free, too. 

use a torrent client to download .. AVG or Avira.. its free.. its not illegal but frowned upon.  :)

 Microsoft security essentials is great. Just uninstall the other one first before installig this one.

Thanks for the input guys. But i am guessing that ms essential is free. So is it any good?
 That is my major concern. i don't mind paying extra 20-40 bucks and want something robust.

There are numerous free-good and legit antivirous out there: AVG, Avast and Avira.. Not sure which is best. I use AVG, works better than Nortan as I already expirenced Norton before. 

Check this out http://anti-virus-software-review.toptenreviews.com/

In the past, i have found norton 360 pretty good. It's less resource hungry but quite effective. Kasperasky used to be gud but nomore... Everybody gives high marks for bitdefender total internet version but I didn't like it that much. I had bought the value version of that particular s/w (.. basically bought that particular s/w which was valid for 2 years and i was legally allowed to install in 3 different systems). It does has a solid track record on virus detection and removal but that thing used crashed a lot .. a total pain in butt.. I may be an isolated incident :( .. Now i use AVG internet security 2012.. well does it get the work done? absolutely it does get the job done.. is it worth paying for the professional version.. i don't think so..So, if u don't wanna go through the hassle of downloading the trial version of AVG internet security 2012 and going to youtube to find the serial keys and changing parameters under  the options tab(u have to uncheck the option that says update the program..but the leave the option check that says update the virus definition.. otherwise u will end up with the free version that lasts only for 30 days ); u r better off buying legit version..if u r bank of america customer they provide u free download of mcfee which is pretty decent s/w.. follow this link http://www.bankofamerica.com/privacy/index.cfm?template=mcafee_internet_security  ... or if u don't believe in free stuff  :) then these s/w are gud price after rebate.. http://slickdeals.net/permadeal/79476/newegg-symantec-norton-antivirus-2012-3user-free-after-40-rebate-tuneup-utilities-2012-3user
http://www.staples.com/Trend-Micro-Titanium-Antivirus-2012-1-User-Boxed-CD/product_368177?externalize=certona

These r gud prices.. the main thing is decide what u need and how much of a computer nerd u r..  and yes dont' forget to install free version of MALWAREBYTES ANTI-MALWARE.. it's an absolute must.. costs u nothing and do run this antimalware couple of times a month ... install ccleaner too.. trust me helps to optimize ur computer/ lappy.. to download free legal s/w go to filehippo.com ..

hope this helps.. and sorry for महाभारत

You know that Comcast internet provides Norton Security, if you or  your friend has Comcast : download Norton free. Which Never expires.

 Thanks people. Really appreciate your help. I got avg finally.