1. First off, Install the game to the Hard Drive from the installation disc.
2. Once that's done, tell Steam to install the game, but pause the download as soon as it starts.
3. Find the directory that Steam's installing to, and copy all the files from the install you made in step 1 to the Steam install directory. (If there's an uninstall executable or Install log file you don't need to copy it over.)
3. Restart the download in Steam. It will now only download files that are either missing or changed. (Steamworks files or updates, mainly)
4. When it gets near the end, I've found it occasionally stalls... (As Lobsang stated, usually at 99%...) If this doesn't happen, skip ahead to step 5. If this DOES happen, follow these steps...
4.1. Create a temp folder on the same hard drive as your steam installation.
4.2. Make sure Steam is still downloading the game (Doesn't matter if it's suspended...) and move all files from the Steam installation folder to the Temp folder you created in Step 4.1
4.3. Now if you go back to the Steam installation folder, you'll see a few files are still there. These are the files that are causing Steam to stall...
4.4. Close Steam And delete the offending files from The Steam Folder AND from the Temp directory you moved everything to.
4.5. Move everything back from the temp directory to the Steam Installation directory
4.6. Restart Steam, and Steam should now download the files it was stuck on from scratch.
4.7. If it should get stuck again, (Personally I've never had this happen, but it could theoretically happen, follow steps 4.1 - 4.6 again... If the download completes, Head on to step 5...
5. Once the download completes, Go to your Games List, right click on the Game list, and Select Properties. Click on the Local Files Tab and click on 'Verify Integrity Of Game Cache'
6. Once that's all done, your game should now be fully installed in Steam...