Infections to your computer are sadly things that are going to continue to happen so the best form of defense is to know the enemy.

There are five types Viruses : this is a program that infect its host and then replicates itself however in order for it to run the user needs to unwittingly start it.

Example if a virus hides itself in a email link or attachment the pc wouldn’t be infected until the user clicks on the link or opens the attachment.

Worms: if your computer is getting slower then it’s a good chance you have a worm these don't need any interaction to run. They are designed to take advantage of security flaws in the operating system.

Trojans:  this is a program designed to look like something else the only way to avoid these is to be careful what you are downloading as it can bring other infections hidden within it.

Rootkits: this is a nasty piece of code often installed via a Trojan it allows other users to access to the inner workings of your pc.

Spyware:  this is potentially less damaging than the others but it is a serious threat to your privacy.  It gathers info about your computer use which it then repots back over the internet.