While a true assessment of Windows' vulnerabilities, you have to take on some level of responsibility. These viruses don't just automatically appear for no reason.
You need to keep your system up-to-date with service packs and be responsible with opening of email attachments. Even further, you shouldn't use the preview window in Outlook (if possible, don't use Outlook or OE at all) and you should disable opening of HTML based emails. You should double check your virus scanning settings. Does it scan all incoming emails? Are you sure? Most of today's viruses spread using known (and patched) exploits.
I agree that Microsoft is partly to blame with all the holes left in their software programs, but if you are going to be a Windows user, you need to know how to change your ways. Running a virus scanner and firewall doesn't make any system invincible. It sort of like blaming a car manufacturer for your dead car at 80,000 miles when you don't do any preventative maintenance.
For kicks, I am curious which 4 viruses turned up on your system? A quick lookup on your favorite virus defs website will show how you transmitted it and where you are lacking protection.
With all that said, I am still a happy Mac user 😀