Re: Windows code 43 with 590 GTX
I'm going to assume you uninstalled the drivers for the 4750's and ran a driver cleaner on those as well? They shouldn't be causing this issue, but you never know sometimes.
As for the new card not working on the newer system, it's possible that the board may have issues with it in the current slot. Since it looks like you had a CrossFire configuration before, what happens when you plug the 590 into the other PCIe slot? It'll probably do the same thing, but it never hurts to try just in case.
Another thing you'll want to do is reset the CMOS on the motherboard, and, in an extreme case, see if a BIOS update is available that fixes this issue. The 590 is a beefy card, and it draws a TON of power. If there's any old configuration data on the motherboard in the BIOS, etc, it could cause hangups.
Failing that, you can install Windows to another hard drive and try running the system in a bare state and see if that isolates the problem. Could be Windows itself too.