You really need your own thread.
First off, I hope you still have the original files on the old computer. You may need to start over.
Were the files on the C Drive originally? In the My Music folder? That is the default for iTunes, I really hope so. I am assuming that your original iTunes was in C drive My Music\iTunes folder unless you say otherwise.
Second, you can remap your My Music folder from the C drive the the D drive with Windows. I did this to save space on my SSD.
1. Locate your Command Prompt shortcut (usually Start->Programs->Accessories->Command Prompt) and right click it and click on Run as Administrator. Click Yes or OK to get authorize the command.
2. Open your task manager (right click on the Task Bar, click on Task Manager) and find the process named Explorer.exe, right click it and choose End Task. If you have more than one, then repeat until they are all gone. Yes it is normal for your Start button and Task Bar to disappear. Close the Task Manager.
3. In the Command Prompt type Explorer.exe and your Start Button and Task Bar will appear.
4. Open a Windows Explorer, locate the My Music folder, right click it and choose Properties.
5. Click on the Location tab, click on the Move button, navigate to the D drive, and create a My Music folder and click Select Folder. In the Move Documents window click No.
Now copy the iTunes folder from the old system to the external drive, then move the external drive to the new computer and copy the iTunes folder to E:\My Music.
If you start iTunes, (and my assumptions were right) then you should see your library, playlists etc. Authorize the iTunes, and play a few tracks to make sure all is right.
Don't forget to deauthorize the old system when you are happy with the new one.