Are we talking about software crashing (i.e., program shutting down unexpectedly), computer freezes where it becomes unresponsive, or computer crashes (i.e., blue screen of death)? I've only gotten those blue screens of death on versions of windows older than XP and maybe once on XP. Freezes happen as soon as I open too many things at once on a PC. I haven't had freezes on my mac. Software crashes I get on both, though with less frequency on my mac than I did on the PC, and yeah, pretty much limited to the browsers and Flash intensive sites.
Though, I'm not pleased with something in my current Mac...they've apparently monkeyed around with the video chips and something is not working right when running Snow Leopard and connecting to an external monitor for several of us who got new mac laptops around the same time. Our IT guy hasn't found a fix, and doesn't think there is one yet, but said he's found that a lot of people are having the same issues. It's the first time I've had a real problem with a Mac, but then, I also don't usually upgrade my OS so early after release for any computer. This time I did because it provided better functionality between Mail and our university mail servers so I could stop using Entourage, which I despise (that's supposed to be the Mac version of Outlook, but it's not and loses a lot of the functionality that Outlook offers).
I do suspect a lot of it really has to do with hardware too, and not necessarily Windows. I've seen some people with PC laptops that really don't have issues. The problem I have is I work for a university, which means I have to order computers from the places we have educational contracts, so basically, they force everyone to buy Dell P(O)Cs. The hardware is horridly unstable in those. If they actually let us buy quality hardware, the PCs would probably be comparable in cost to Macs, but for now, our choices are Dell or Mac, so of course it makes sense to go with Macs, which usually work better than Dells (one new video chip notwithstanding).