First, copy all your personal files (music, photos, scans etc) to your external hard drive. You should do this anyway as a "backup".
There are all sorts of methods of getting files from your PC to your external hard drive such as drag and drop, copy and paste, or "Send To".
Then have a look at Windows Easy Transfer which should help you. Here is the Windows 7 one but there are others for other versions of Windows.
http://windows.microsoft.com/en-GB/windows7/products/features/windows-easy-transfer