The endless hours of virus scanning, cleaning, reinstalling applications (which is going on as I type, oddly enough) to correct for Windows vulnerabilities, never seems to register to Windows defenders.
If you're spending endless hours scanning for viruses, I'm forced to wonder if you're not intentionally installing viruses just so you can scan for them. I'm not saying you are, but doing regular scans for viruses, let alone countless hours, is simply not something that people do because even free virus scanners like AVG will intercept virus activity as it comes in. I don't think I've ever performed a virus scan on my personal system and in the IT systems I've managed, I never performed a virus scan on any workstations or servers because they all had background processes which stop that kind of thing from happening. I've never, ever had an infection.
The only reason to scan an entire system for a virus is if you're installing antivirus software after you think you've been infected, in which case you should just clean up and start over with antivirus installed first thing. Regular virus scans aren't really something that has been done for over a decade, and even then they weren't "constant."
As far as vulnerabilities go, I get a notification that updates are available, I click install (when I feel like it), it minimizes, and I continue with what I'm doing. I don't even notice it.
Reinstalling applications? I've never had to reinstall an application except when the application itself broke through either it's own internal update process or I've received instructions from the application vendor to do so for troubleshooting -- never anything to do with Windows. The only time I ever go to the Add/Remove Programs applet (now Programs/Features in Vista) is when I don't want to ever use an application anymore and want to uninstall it.
When people say these things about Windows (constant virus scans, constant reinstalls of components to secure vulnerabilities, constantly reinstalling applications), I really have to wonder if they're not using some imitation OS brand that says "Whindose" or something, because these things simply don't happen -- or if they're still using Windows 98 and someone painted "XP" or "Vista" on the box without telling them.
Another major cause of constant reinstalls, constant virus scanning, and related activities is something people in the IT field call "voodoo troubleshooting" because the cause of an issue is not known and so assumptions are made (must be a virus, must be an update, etc.) -- especially since the entire tech support field (especially telephone support) has been filled with unqualified people who tell end-users to reinstall or virus scan for every problem under the sun.