The problems with spyware is not "annoying windows" or "viruses that freeze your computer"

The problem is that spyware will send private information to who-knows-where without affecting ANY performance (many times it does but not always). This could be trackers which track your internet activity, malware which could send passwords, etc...

So the problem is not "where does your computer slow down that you need ant spyware" but the point is that you don't know its there... probably ever.