Writing off the "whole XPC range" just because of this problem is a bit like never again flying on a Boeing simply because Southwest Airlines lost your luggage.
It's possible you have a bad mobo/system. It happens, occasionally. But it's also possible something else is wrong. As emkubed said, you need to narrow down the problem.
and run several passes. If you get even a single error, there's a problem. If you're using more than one stick of RAM, take it down to one and then the other to determine which one is at fault. Also try different RAM slots -- sometimes that can be the problem.
Try a different SATA cable. The pre-routed ones are nice, but it's possible it got kinked while the box was being built. Double-check the connections: some people have reported their SATA cables are prone to working themselves loose.
Try a different optical drive and cable. It's possible Windows is getting corrupted as it gets read off the CD.
Have you looked at your CPU temps? Particularly right after an install or running memtest? Small cases tend to overheat very easily, and if you didn't put the heatsink on perfectly you could have a problem there.