You need to add your music collection to the iTunes Library as follows:
1. Launch iTunes (on your new PC).
2. Click File, then click 'Add Folder to Library...'
3. In the 'Browse For Folder' popup window, navigate to the folder containing your songs and click OK; it will take a few minutes depending on how large your music collection is but you should be good to go from there!!
I do this all the time...
Note: you can also add individual songs to the iTunes Library by clicking 'Add File to Library...'