Try selecting Preferences under the Edit Menu, and then selecting the "Advanced" icon in the upper right of the preference dialogue.
Beneath the path statement that identifies the location of your current iTunes Media folder, you'll see two checkboxes labelled "Keep iTunes Media folder organized" and "Copy files to iTunes Media folder when adding to library." Be sure that a checkmark appears in both boxes.
Click on the "Change" button to the right of the "iTunes Media folder location" display to open the popup screen. Use the directory tree to browse to the default /music folder on your NAS drive. With the default /music folder highlighted, click the "Make New Folder" button in the lower left of the window, and enter the same name as you current iTunes Media folder. (Depending on your installation, this may vary, but it's probably best to replicate the name of the folder on the drive where iTunes is currently storing your media files). Be sure your new folder is highlighted, and click OK.
iTunes will move your media files to the NAS device, and continue to manage and organize them there. (iTunes must remain open until the move is complete.) Your PC's registry should be updated to reflect the new location, so the change should be transparent to relevant applications on your system.
Note that iTunes may become unhappy if loaded whenever the NAS device is inaccessible to the computer on which iTunes is installed. Inability to access your newly created iTunes library may cause iTunes to create a new (empty) library in a default location, (which can vary depending on the version of iTunes installed). In light of this, it's probably a good idea to edit any shortcuts to iTunes on your system by specifying the new media location as the start directory for loading the iTunes app. You can do this in Windows XP, Vista & 7 by right-clicking on any desktop icons you may have, (as well as any line items in your Windows Start Menu), and selecting "Properties," where you can enter the full path to your new Media folder in the box labelled "Start In." On Windows 8+, you can hold down a desktop tile and select "Open file location." On the resulting explorer screen, the shortcut to the executable should already be checked and highlighted. Hold down on the shortcut's highlighted filename, select "Properties," and proceed as indicated for versions XP -- 7.
Once you've specified the new library location in the shortcut properties, iTunes will bypass any previously stored media location defaults and zip right on over to the library on the NAS device. If that device is ever inaccessible, you'll be alerted with a warning message whenever you load iTunes from a shortcut specifying the NAS directory.