Get a PC without HTC drivers and stuff, download the Universal Naked Driver (google it, it's on XDA), extract it somewhere. Connect the device to the PC, and let it search manually for the drivers in the folder where you extracted the UND. It will install the driver. Then get Adb, and use 'adb reboot recovery'. This has to work.
Also, another test: make sure the device is connected to your PC, and keep Vol Down and Power pressed for a minute. If you heard any sound from your PC regarding a USB connection (either unplugged or plugged), you know that you triggered it into HBOOT.
HTC HD2 / Nexus 7 / HTC Flyer / HTC Desire
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