An interesting problem but should be easily fixed. However, I'm unfamiliar with Vista so this would be the fix for XP, though I can't imagine Vista could be too different.
These things we know;
- you're iTunes library exists.
- iTunes is installed correctly on the new laptop.
1. Boot up Vista and then make sure your external harddrive is turned on
2. Make sure your external harddrive is showing up in Windows Explorer
3. Open iTunes, choose Edit -> Preferences -> Advanced tab (this is where your iTunes Music folder location will show)

4. click the 'change' button - this will open a 'Browse For Folder' window

5. Browse to your external harrdrive letter and click on the iTunes folder where your library is. The image in the browser window should then show an open folder - mine is sitting on Partition E: and is just called 'iTunes'.
I'm pretty sure that's how easy it should be.