I have a HTC Desire that suddenly started going into a reboot loop, did a full factory reset, upgraded the firmware etc, in the end I gave up and tossed it in a pile of other crap I have, apparently the motherboards on the HTC Desire phones have hardware issues that can cause this reboot loop, one I heard of was a faulty temperature sensor, haven't been arsed to look into it and probably never will, but might be a hardware issue like mine is.
Mine gets to the white HTC logo and then reboots, and occasionally (like 1 in 25 / 30 tries) it gets to the main screen then reboots again.