I worked in the IT industry for a few years and have owned and maintained many PCs in my time.
I got sick of having to "maintain" the things every couple of weeks. The applications' authors didn't write proper uninstall routines, so loads of junk was left over even after you thought you'd removed something. There were so many viruses and malware it was a joke. Hence, you have all this extra anti-virus/spyware software installed that you shouldn't really need clogging up your system and using resources.
Another problem is that when you install Windows (and this applies to OS X too), is that the default user is an Admin user, and this gives them complete power over the machine. This account should never be used for everyday use.
Any viruses you download then also have that Admin power. It's better to create an Admin user specifically for software maintenance etc, then create limited accounts for everyday use by you and your family. It will limit the damage any malicious software will be able to wreck.
PCs are not the problem - the hardware is fine - the trouble is the god-awful Windows operating system, which has been inherently flawed for years. Up until recently it depended on the underlying MS-DOS which was a clunky single-user operating system. Security was never a factor in its design.
OS X on the other hand, is based on a Unix variant, as is Linux, and these systems have been security aware from their inception. When I was at Uni and the internet was still unheard of except in academic and military institutions, Unix was used for good reason. It's very secure, and is as old as the Internet itself.
So, I use a Mac now, and have never been happier. Underneath the pretty user interface is a powerful Unix system if you need it!
