Just to give you a comparison:
OS X - Been using my Mac for about 7 months now. In that time period, i've yet to have a kernal panic (unrecoverable OS crash). Apps however, are a different story. Safari seems to be the main suspect, crashing once every few days, which is more than i'd like it to. Also, iChat can act a bit funky at times. Although it doesn't crash, after a few days of being open, it suddenly decides to take up 100% of my CPU time. I have to restart iChat to bring it back to normal. Also, back when Panther was first released, I had horrible issues with Finder crashing, and icons getting corrupted.
Windows XP - been using it since it was released back in 2001. In that time, i've yet to have a hard OS crash (blue screen of death). However, apps do occasionally crash, although no where with the frequency that i've seen in OS X. I honestly can't remember the last time something crashed. Maybe it was Firefox, but then again, its beta software so its to be expected.
Now, I take care of my Windows system. I've been a windows user for a *long* time, and I know the inns and outs, and what not to do. This is probably why i've been trouble free with my computers. The same can't be said with my friends. They constantly come running to be with the strangest problems - although its usually in the form of Spyware/adware taking over their computer. This isn't a problem with Windows stability however, its a problem with windows security, and user idiocy. If you're running "screensaver.exe" files that you're getting from unknown emails, you deserve to get that virus, IMO.
Windows also tends to "slow down" after a while. But again, this isn't a problem with windows directly, its an issue with users installing a ton of garbage that forces itself to load on bootup and sit in the system tray.