One of the most common forms of virus is adware and if you believe you have adware the first thing we should do is make sure your troubles aren't the result of normal every day adds personally I use Firefox and with that I install a few addons the first is Ad block plus the second is Element hiding helper the third is a Pop up blocker and finally is Kill jasmin these 4 addons should eliminate any adds that annoy you see also RIP If by then you are still seeing pop ups you might have a virus . Defeating a viruses however is a lot easier than you think the first step is to have a good personal firewall .If you have to use something free I suggest Microsoft security essentials along with SiteAdvisor search and webutation as well as Wot PS AVG ,Avira and Avast are the worst do yourself a favor and never use them they are plagued with false positive and negative readings they even alert on uninstall wizzards and wont protect your pc from anything they are essentially a waste of memory and ram .Having a site adviser is key to preventing a virus as well because it will warn or prevent you from downloading an infected file in the first place .The best anti malware which should not be confused with a personal firewall in my opinion is malwarebytes and if a virus prevents you from using it you can use cameleon to help get malwarebytes on your system,and remember to update malwarebytes every time you use it .Also very good is Kapersky rescue disk when a virus doesnt allow you to boot to desktop .Another really good method for ridding your computer of viruses is to perform a Windows 7 system restore Windows 8 system restore finally XP restore and restore to a point prior to infection. Finally remember you can only have 1 personal firewall ,but that doesn't include a site adviser or anti malware .All these programs are compatible with each other and you do not need to run in safe mode . Finally also remember that if you have a firewall simply going to a site any site will not infect you it's when you run a infected programs especially ones from torrents that you will get infected .PS also keep in mind even if a program is virus free read the install agreement freeware providers will often try to slip in bloat ware or change your home page or search engine that is not virus behavior but can be annoying if you don't know how to undo such changes installing software in custom or expert mode will prevent such things from occurring If all else fails you can consider restoring factory conditions by reinstalling your operating system .If you have the installation disk simply boot with it and follow directions ,but if you do not you might have the option to use a built in back up how you do that however depends on the pc model and operating system to find out google the words restore then model and operating system to factory condition a very good blog for this is Ehow .
