If they're finished, the easiest way I can think of would be to "share" each one of the houses so it creates a file for them in your "exports" folder. Then copy them to a flash drive and transfer those files to your "imports" folder on your new computer. You can then use the game launcher to place the houses to your empty houses bin.
However, if your old computer has too many issues or if they're not finished yet, it might actually be easier to simply copy the saves of them and transfer them to the saves folder on your new computer. You can then extract the houses by any means you see fit, or continue to work on them.