Meghana, the Dell people told you quite right. The only catch is that the company that markets it -Canonical, is like MS of the Linux World. (Yeah, I know there is RedHat!) They give you the OS for free, but there are a number of applications that you are required to purchase from Canonical/others. There are free and opensource altenatives to that as well.
As far as virus infection is concerned, it has to be less prone to viruses in comparision to Windows (the largest used PC OS in the world). These people (virus makers) don't quite target an OS which has few takers.
Till a couple of years back, most of the hardware manufacturers didn't pay quite an attention to creating Linux Drivers, but that seems to have been resolved now.
And the pros are now for the manufacturers who save their licensing fees for propreitory OS's like MS Windows.
As for the Antivirus, you can try the free ones like Avast/AVG for your linux installations. For cons, there are quite none except that if you've known computing on windows, you'll miss a lot of applications.