Start over from scratch. If that means going step by step through the setup process for your wife's PC, then do it.
Unless there's some interesting software involved that you didn't mention, the Mac and the PC should both have their own, unique IP addresses. Also, either enable DHCP on both, or manually give both an IP in the 192.168.1.X range.
Depending on what PC operating system you're using, you may be able to get away with controlling it remotely, but the odds are against a stable situation. Windows isn't too great sometimes with hardwired console peripherals (keyboard/monitor/mouse) much less with "virtual" components.
