I know what you mean, I work in systems support and my wife complained to me last month that her new laptop was slow. I loaded a spybot and found over 250. No viruses, I keep nortons on my machines for that.
A few months ago, I gave my son my "old" computer; a machine that had always performed perfectly for me, and which had a good antivirus program that I kept updated, as well as downloading all of the MS updates and fixes. Last month, they began to complain of it crashing. Then it stopped altogether. I ran AdAware and found over 700 instances of viruses, spyware and other data miners and dirty tricks. But they still had to reformat and reinstall. Files were so corrupted they simply couldn't run. Of course, neither of them ever thought to install any kind of firewall.