Addware & Spyware -- What They Do To Your Machine.
Malicious software like spyware and adware can be annoying in a number of different ways. Programs differ in terms of what they do to your machine, going from things that just slow your computer down and serving advertisements, up to more damaging things that can stop your computer working at all.
First off, the #1 thing all spyware and adware does is slow down your computer. In order to do things like display advertisements (as with adware) and spy on what you do online (as with spyware), software running in the background will need to take up some of your computers resources. This leaves less CPU and RAM for other system processes, so it may take your computer a longer time to do things.
Once this happens, your computer may take obscenely long times to open new software or start up. In addition, your computer may end up locking up completely if you have too much is running. The problem increases with the more spyware you get. Thus, if your computer is getting progressively slower, then it is a good chance that it is spyware that is doing it.
The second thing spyware does is relay information .This puts your information at risk, which makes it much more damaging. In fact, identity theft most often comes from criminals who use spyware to record your information.
If you use the internet for anything much, then malicious programs can easily record sensitive information. People who make malicious programs and record information like this often sell it to other criminals, who use it mostly for identity theft (and occasionally for money laundering). As you can imagine, this is the most damaging kind of spyware.