1. Update your client to 1.4
2. Download a 1.3 client with \bin files, or get one in your second computer.
(I can host one but not sure if its legal to do that here)
3. Go here ; C:\Users\Your-Session-User\AppData\Roaming\.minecraft\bin
4. Delete the new minecraft.jar (updated to 1.4)
5. Rename your old minecraft-1.3.01.jar file to minecraft.jar
6. Keep the 1.4 Version file
I just did that and work fine for me
Later, when you want and are ready to update to 1.4, just delete the version file or \bin folder and reboot.