Plug in your exterrnal USB drive on the old computer.
On your old PC's 'C' drive, locate your iTunes library. (Documents and SettingsusernameMy DocumentsMy Music)
Right click and click 'copy' on the iTunes library icon on your C drive.
Now go to 'My Computer', open the external drive, right click in it and 'paste'. The iTunes library will be copied to it.
When it's finished plug the external drive into the new computer.
Go to 'my computer' and click on 'C' drive. Find... UsersusernameMy Music (Windows 7) or .. . Documents and SettingsusernameMy DocumentsMy Music (Win XP).
In another window open your external drive and either drag the iTunes libray icon or right click and copy.
Drop (or paste) the iTunes library onto your C drive into 'my music'.
When that's finished, download and install iTunes from Apple UK's download website and it will find everything (settings, music files, playlists artwork etc.) automatically.