Usually, bad things on your PC these days aren’t technically viruses, they are trojan horses, worms, adware, key loggers, search hijackers and so on. Generically we call bad things that you don’t want on your PC “malware“.
Most infected users have in fact downloaded the malware themselves, and clicked “OK” on lots of boxes in the process. They do this because the malware installer claims to be something useful (it’s lying). Often people download things that claims to be a Security Scanner, a Registry Cleaner, a Speed Maximiser, a PC Tune-up Manager, a Driver Updater, or a utility that claims to Fix Unreadable Files or Fix Download Problem (or they leave a box ticked that offers a “free download” of something apparently useful. Virtually all of these fake products are downloaded from professional-looking and convincing sites … judging a site by how professional it looks is always unwise. Malware distributors make enough money to be able to afford excellent websites! (Even if these things did what they claimed and didn’t also install malware, they would be pointless. They sound technical and important, but they’re not. For 99 percent of users, registries don’t need cleaning, drivers don’t need updating, and so on.
If your PC is slow, a few simple things you can do yourself will be much more effective that any spurious “PC Tune Up” program.
Another thing to watch out for is where you download legitimate software from. The thing you want (iTunes, VLC, Microsoft Security Essentials, Flash Player) may be legitimate and useful, but are you getting it from the right place? Getting it from the wrong place may mean you download something undesirable as well. Do your research before you download.
And finally, watch out for adverts that look like warnings, and unusual search engines that may look like Google. Don’t trust what they are telling you, especially if they want you to download something.
Good luck out there – keep your wits about you!