1. Get a second computer with the same operating system version which didn’t have a 3G modem installed (for example if your target system is running Windows 7 64 bit you need a second system with Windows 7 64 bit – different skews like Home vs Ultimate are ok, but the version and "bitness" must coincide – you could also try using a virtual machine for the second machine which supports USB forwarding like VirtualBox or VMWare)
2. Plug in the modem in the second machine. First it will recognize it as an USB stick / CD-ROM. Copy all the files from it to a separate folder (you should see files like "setup.exe").
3. Let the setup complete. Now copy the installed drivers to the same place you’ve saved setup file. Under Windows 7 you would find them in the location C:WindowsSystem32DriverStoreFileRepository in several folders starting with "gth" (like gthsubus_64.inf_amd64_neutral_4810563f34b37ef5), but here is the generic way to identify the folder:
   1. Start Device Manager
   2. Look for one of the devices associated with the modem (you will find actually several, like GlobeTrotter GI515M – Modem Interface, Network Interface and so on)
   3. Properties -> Driver -> Driver Details. Note the name of the driver for which the provider is Option (for example gtuhsser.sys)
   4. Now search your Windows folder for files ending in .inf which contain the name of driver from the previous step. This will point you to the right folders
4. On the first computer (the one you actually want to install the modem on) remove all previous versions of the software using the Add-Remove Programs facility (you will see two-three entries but they can be easily identified by the same orange icon). Restart the computer for good measure.
5. Copy over the setup program and the drivers from the second computer. Plug in the modem to the first computer, install the application (using the setup file captured on the second computer). Go into the device manager and look for "Unknown device"s (you should see four of them). Use the drivers captured on the second computer to resolve these issues.
6. Unplug and replug the modem – it now should work!

A couple more talking points:

• don’t use "driver manager" type software – they very rarely (read: never) seem to work
• a symptom that you’ve hit this problem is when the management interface (dialer / "Internet Everywhere") for the modem starts but it gets stuck in the "Initializing" phase when you connect the modem and consumes CPU (from what I’ve seen with a debugger it seems to be looking for the installed device in a loop)
• the modem seems to be prone to overheating if the signal-strength is low (around two bars) and in this case it shuts down after ~10 minutes (I assume that this is some kind of thermal protection). You can check if this is the case by putting your hand on the bottom side of the modem. I couldn’t find and solution for this, other than looking for a spot which has better signal. Using the modem in EDGE rather than 3G mode also seems to do the trick, but it has lower speeds and I don’t know of any reliable method to make the modem use EDGE if 3G is also available.