How do I disable Fast boot without the Power option in Settings? I went over some of the old forum posts about this issue and there seems to be a lot of confusion about this. I am planning to root my HTC Desire (GSM) which at the moment is running original UI/ROM and everything on Android 2.2 FroYo.

One of the root guides said that I should disable Fastboot by going to Settings and then Power. But I don't have that option. And apparently I am not the only one.

So what's this all about? And how do you disable something like that when the option is not there? Can this be done from HBoot menu?...
I believe the they didn't make fastboot an option until Gingerbread. It should be off by default so I wouldn't worry about that step. You can check to see if it's on by turning your phone completely off and then turning it on to see how long it takes to boot up. If it only takes 10-15 second fastboot may be enabled. 

Also, here is a painless way to root your HTC Desire that doesn't require changing fastboot: How to root HTC Desire (Froyo 2.2) without putting in a custom ROM (flashing)?