itunes importing library mass confusion

Hi,

I recently upgraded to Snow Leopard and needed to reimport my itunes library. To cut a long story short, I ended up with my itunes folder twice. Forstly, I had the back up copy on my external drive and then I also have it on my data drive.
Somehow I seem to have imported it (which at 50+ GB took about 8 hours) from my external drive (when I click on the info for a track it tells me this) so there is a delay whenever it finds the tracks and during pausing.

Obviously, I'd rather it was reading it from the Data folder on the hard drive.
Is there anyway of switching the source from the external folder to the data one?

Obviously I don't really know what I'm doing (the up[grade was done for me) so any advice would be welcome!

I'm assuming that I'll need to delete the contents of itunes completely (without moving the actual files to trash) and then reimport from the data drive but, as it took so long last time, I was hoping to avoid this and wondering if there was a quick fix?

Also...

If I DO have to reimport the tracks from the itunes library on the data drive, and this is another point of confusion for me, should I uncheck the "Copy files to itunes library" option in the advanced tab first? I assume that if I don't then the tracks will be copied again and I'll end up with everything duplicated?

As you can tell, this is confusing me somewhat so any help would be appreciated and, hopefully, will save me a lot of head scratching and Googling!

I keep reading about an .xls library file but there doesn't seem to be one in either folder.

Thanks a lot,

Mark