My two cents (naturally).
Windows operating systems are no more vulnerable to virus, spyware and/or crashing than a mac is, the simple fact is that PC's are the majority so in turn they are the target 99.99% of the time. As Macs get more and more popular and common in the home, they too will see spyware, virus's etc. The best defense is to educate yourself about the origins of spyware, virus and general misbehavior and it's causes. Learn how to recognize web sites that are potentially bad news. Being a network security guy by trade, I have absolutely zero issues with any of the Windows based computer I use each and every day, why? Because I know what to stay away from.
Think about this; If you were a malware author that wanted to get his little application installed on peoples computers because every time they click your banner you make 4 cents, what OS would you write the malware application to run on? Hmm, Windows of course, that is if you want to hit the masses. You would most likely skip Macs and Linux because there simply are not as many common users who will fall for your scam that run them on a daily basis.
So, you can spend the money and buy a mac and then have loads of fun trying to find software to work with it, or you can educate yourself for free about the dangers of the internet and how to avoid them.