You might be able to redownload them on your PC via the Purchased link under Quicklinks on the right-hand side of the iTunes store homepage. Or you should also be able to connect your phone or iPod to your computer's iTunes and do File > Devices > Transfer Purchases, that should copy them over.
The 'automatic downloads' page that you posted from only applies to new purchases e.g. if it's now 'on' on your PC's iTunes then if you buy any more music on your iPhone and/or iPod then those track(s) should also then automatically download on your PC