I didn't have much trouble at all switching over to Linux a few years ago.
Anyone can learn it, but you have to set your mind to it and it really helps to have someone who knows about it and can help you out in a bind. It's not something that you can take on lightly, you have to want to learn it.
But if you do want to learn it, then it's not that hard to grasp. Especially if you go with a commercial version like RedHat that comes with customer support.
So, would Linux work well in a PII or PIII machine? Can older machines get new life with Linux?