They allow other people to track your Internet patterns and record your credit card and any other personal information without your permission. Show unwanted ads on your computer. Modify, copy or delete your files. And if it weren't enough they slow down your computer and Internet connection.
You have to be very knoledgeble when it comes to anti-virus software and firewalls because not all of them work, are good and some of them are even infected with the pests you are tring to get rid of. (Usually the free ones, only if they don't come from reputable companies like kaspersky, northon, nod32 etc.)
It it weren't enough just as it is they are keeping on advancing into new tricky and undermining types of web assaults, that's way anti-viruses have almost daily updates and why it's best to have a subscription for one once you find what is the best type of program based on your usage habits.
The most terifing thing is that when they infect a fille you can rarelly save it and even the best anti-virus out there will completely delete your file. From the user who use the computer once a week to store pictures and recordings to the more advanced one whose work is based on the usage of a computer this is a real problem.
It's best to form good internet habits or start changing them. Know what a threat is, where you can get it, how it manifests, what to do and NOT do in the case of an inffection (most viruses will follow you and enter the filles you acces, if you access the contaminated folder then move to another one, that one will ALSO become infected) but most important, how to avoid a nasty situation and enjoy a safe and fast computer and internet connection.
Buckle your belt and be ready to make some changes. It's beter to have an umblera with you if you know that it may rain today. Especially if you have important filles on your PC. I know that I have and I guess that you have too.