In order to use the HTC tattoo as a USB modem with your Windows 7-based PC to connect to the Internet, firstly set the Tattoo to Disk Drive Mode before connecting it to your PC for the first time.
Within the menu tap on the USB to PC icon:

Set the option to Disk Drive and tap done:

Connect the Tattoo to your Windows 7 PC. The device will be detected as a USB disk drive and the contents will be displayed. Install the HTC Sync application.

Once complete, return to the USB to PC setting on the Tattoo and adjust it to Mobile Network Sharing mode:

Windows will detect new hardware and attempt to install a new Network Adapter. The installation procedure will fail. Don't Panic.

Click on the Start Menu and right click on the entry for My Computer. Select Properties. Open the Device Manager:

The Tattoo will be listed as an Android device in the Other Devices section. Right click on the entry and select the option to Update Driver.

Select the option to Browse the computer for driver software.

Browse to C:\Program Files \ HTC \ HTC Driver \ Driver Files \ Vista_x86

Click OK. Windows will now install the correct driver for the new network adapter:

Once installed, you will be prompted to enter a location for the new Network Connection.

You will now be connected to the Internet (assuming that the Tattoo is successfully connected to your mobile operator's Internet service)

NOTE - I found I had to disable my internal network adapter before the Tattoo's connection would work successfully, but that may have been due to an IP address range conflict between my local network and the addressing scheme used by the mobile operator.