[G1 Q&A, Help & Troubleshooting] frankencat:
I don't know if this has already been answered but you can do this using FASTBOOT. All the info is here @ http://developer.htc.com/adp.html

The biggest challenge is getting the USB driver working. You can search on here for that info but basically you need to install the Android SDK and use the USB drivers supplied instead of the standard Windows driver. After you attach your device to your computer you look in Windows Device Manager and find your phone among the various USB devices (do this while in FASTBOOT mode). The phone device will probably have a yellow exclamation indicating some conflict but you can check to see if it is the correct one by looking in the properties for that device and it will say "Android" etc.. Once you have found it you need to update the driver and install the specific driver that comes in the SDK under usb_drivers. After that you should have no more conflicts in Windows Device Manager and your device should show in the list when you run "fastboot devices". If that works then you can continue...here are the steps that I followed (from the HTC dev site)...

First make sure you are in FASTBOOT mode. Do this...

* Get the phone into FASTBOOT mode in the boot loader screen (press CAMERA+POWER on startup and hold until white boot loader screen comes up).

* Connect the device to your computer via USB (if you haven't already done so) and alternately use the CAMERA and BACK buttons to switch the desplay between "Serial0" and "FASTBOOT". FASTBOOT is what you want.

Then follow the steps fro mthe HTC site...

Flash the System Image Package to the Device

For the fastboot method only, after you've successfully updated the radio firmware, the next (and final) step is using the fastboot tool to flash the system image package to the device. Note that you should always update the radio image whenever you update the Android system image on your device. (If you are using the recovery-image method, you do not need to flash the system image package to the device - see Copy the Recovery Image Package to the Device instead.)

Follow the steps below to flash a system image package to your device:

1. Boot the device in fastboot mode.

To enter fastboot mode, power up the device (or reboot it) while holding down the BACK key. Hold the BACK key down until the bootloader screen is visible and shows "FASTBOOT". The device is now in fastboot mode and is ready to receive fastboot commands. If you want to exit fastboot mode at this point, you can hold down the keys MENU+SEND+END (on the Android Dev Phone, SEND is the "Call" key and END is the "End call" key).

Note that the bootloader screen may vary across devices. For Android Dev Phone devices, the bootloader screen shows an image of skateboarding robots. Other devices may show a different image or color pattern. In all cases, the bootloader screen shows the text "FASTBOOT" when in fastboot mode. The bootloader also shows the radio version.

2. Connect the device to your development machine over USB, if it isn't currently connected.

3. You may want erase the contents of the userdata partition, by using this fastboot command:

$ fastboot erase userdata

This will remove all of the installed applications and their data, but it is a useful step when debugging because it ensures a clean execution environment for the application. Erasing the user data may also be necessary when you are flashing a lower (downgrade) platform version to your device. However, it is not strictly necessary to erase the userdata partition when upgrading to a higher (upgrade) platform version, which is the typical use-case in consumer devices.

4. You may also want to erase the cache partition, using this fastboot command:

$ fastboot erase cache

Note that, rather than using separate fastboot commands to erase the userdata and cache partitions, as described in the previous steps, you can simply supply the -w option when flashing the system image, which has the same result.

5. Next, use this fastboot command to flash the contents of the image package to the device:

$ fastboot update <imagepackage>.zip [-w]

With this command, fastboot copies all of the images in <system-image-package>.zip to the appropriate partitions on the device. During the process, it prints status messages to the command shell. Depending on your bootloader version, fastboot may also print status messages to the bootloader screen.

When all image are copied to the device successfully, fastboot displays a success message.

For information about other fastboot commands, use

$ fastboot

6. Reboot the device. In most cases, fastboot should reboot the device automatically after copying the image files.

If necessary, you can reboot the device manually using these fastboot commands:

$ fastboot reboot

or

$ fastboot reboot-bootloader

You can also reboot from the device by holding down the key combination MENU+SEND+END.

After rebooting, the device should start normally. If the device does not start normally, you can try flashing it again. If you did not erase the userdata partition and the device hangs during boot, try erasing the userdata partition and then reboot.