I have a IBM T40p Thinkpad, running Windows XP Pro. No antivirus, no anti-anything. Just with a firewall and careful usage habit. It's been running like that for about a year now, it has not crashed once. That's a machine from beginning of 2003. It has 512MB Ram, I always wanted to upgrade that.
I have another iMac G4, from i believe mid-2004. With 1GB Ram.
The iMac runs noticeably slower than the IBM in various day-to-day tasks such as web browsing (which include online music video), iTunes, MS Office.
Sure, that's just my single experience, cannot be used as generalization, blah, blah. But that kind of prove what you said false.
Actually, I have a couple of Thinkpads at home - the X series. They're great. I used to work for IBM, and the Thinkpad they gave me I sent through war. It still worked fine.
The Thinkpads around here at work people abuse, but they still work great also (I have an old T30 that the other division was disposing of that I'm using as a backup). And yes, Intel processors run cooler and faster than PPCs...