The easiest way is to copy them over via something like a USB memory stick, CD/DVD, etc. Then merely import them into her library.
However, if you're able to I would suggest doing this.
Copy ALL of your music, videos, etc. over to your wife's Mac.
Set up a "shared" drive or folder with the Mac as the server. Then 'share' the iTunes folder.
Now on your PC, connect to her shared folder. It will most likely come up as a "drive". Once you do that, make sure you map it as a drive letter for Windows. Then hold down the SHIFT key and double click iTunes. Click "Choose Library" and then point it to the new drive letter.
After this, you will both always share the same library. So if she adds something you'll see it automatically. And the same goes for you.