That's why I use Macs - even if my time's worth $20/hour (the lowest rate I ever bill), an extra hour per day dealing with Windoze thuggery would pay off a very nice Mac in 3-4 months...
-Dan
/shrug
I use a free (Avast!) AV suite on my PC. I've been using personal computers of one sort or another since 1981. I've never been infected with a virus, trojan or anything else. Sometimes a spam email comes in and the AV gets a hit, but I never open them anyway.
There's absolutely no time needed to deal with these issues unless you enjoy opening random emails of dubious nature or visit odd websites. It's user behaviour, primarily, that dictates wether you will be at risk.
At some point, if Macs get enough market share, they will likely become more targetted but just a little thought about what you're doing will protect you on either platform, with a little added protection for free if you feel like it (some corporate environments have little choice but to open random emails, although using something like Google Apps for your email can add a good layer of protection without the need for server level AV which most small business might not be interested in).
It's utterly a myth that PCs are at constant risk or require any sort of constant maintenance anymore than a Mac does.