Tje Creatove SB Xfi Sound card is NOT your Computer, just the sound card that is in it. likewise the RealTek in the others.
First thought, reinstall (or install) the correct sound drivers for each system. e.g. the SB Xfi, make sure you are using the correct Windows 7 specific drivers from Creative. Same way with the systems with RealTek, install each PC Vendor's RealTek Win 7 driver. The vendor drivers work best.