How do I disable Fast boot without the Power option in Settings?
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)?