Almost all antivirus you can download/purchase online. No need to have a cd/dvd drive. You just need to know the name of the antivirus and download the program. Paid antivirus you will need the key from physical or online purchase
If your not downloading a lot of things and visiting to shady sites, then a free antivirus is more than enough protection for a home user.
They are three free antivirus that people use. AVG, Avast, and Avira. Avira is apparently the best at virus detection, but the most on having false positives. AVG is a popular antivirus back in the day but Avast has taken over the throne. Most review sites have reviewed Avast and have found it to be in par with some of the paid antivirus. Making it the best free antivirus, but if your a careful computer user then it shouldn't be a problem with other antivirus. Other then those three, Microsoft security essential and Panda Cloud antivirus are free cloud antivirus program that are decent.
Paid antivirus if your very paranoid of virus or you want strong protection against them. Bitdefender is the best one in 2016 I believe from reviews and the independent antivirus testing:
https://www.av-test.org/en/antivirus/home-windows/tl:dr.. for free antivirus Avast is a good choice. AVG and Avira are also good (honestly preference). If you want the extra security get Bitdefender
Overall free security programs can just as well protect you from virus as paid security software
Here is a review site that has good review on free software (software, utility, multimedia, ect)
http://www.techsupportalert.com/best-free-windows-desktop-software.htm