We're going to show you how multiple users on a shared Windows computer can merge their iTunes libraries. This tutorial assumes that the users on your computer are set up for administrative access, which allows you to see and modify each others' files.
Login as the user with the smaller of the two iTunes libraries. You'll need to back this library up, so picking the smaller library will save you time.
Let's back up your current iTunes library. In iTunes, go to the File menu, highlight Library, and select "Back up to Disc".
Keep in mind that the backup process will copy all your iTunes content, including iPhone applications, audiobooks, games, movies, music, music videos, podcasts, ringtones, and TV shows. If you have any content you don't care about taking with you (do you really need all those old podcasts?) delete it before you waste your time backing it up.
The iTunes backup dialog offers a few different options, but for our purpose we're going to leave it on the first default option and back up your entire library. There are ways around doing this, but it never hurts to have a disc backup of your iTunes library in an emergency and the disc method offers the cleanest, most seamless, back up method.
Depending on the size of your library, you may have to back up your collection across several CDs or DVDs.
Look in the My Music folder of the user whose library will be acting as the master library. By default, you'll find it in "C:\Documents and Settings\USERNAME\My Documents\My Music\iTunes\." Within the user's iTunes folder you should find an iTunes Library file. Select it and hit OK.
Open up iTunes Preferences, check under the Advanced tab, and make sure the Music Library path points to the new location you selected in step 5 (it should). Also, check off that box labeled "Copy files to iTunes Music folder when adding to library." This will make sure any new music you add will spill into the new location. Hit OK.
Now, with iTunes still open, insert the backup disc you made (or the first in the series of discs, if your collection was too large for a single disc). iTunes will ask whether you want to restore from this disc. Click Restore. Finally, hit "Consolidate Library" in the file menu just to make sure everything gets copied to the new location. iTunes should remember all these changes from here forward, without you making any extra effort.
Perform these same steps on any other accounts you want to merge on this same computer and you'll have one big happy iTunes library filled with everyone's music. The only catch is that different users can't run iTunes simultaneously, so be sure to quit iTunes before you log off.