Try deleting "C:\Mach3" folder and installing a fresh copy of Mach3 without your old XML files. Then try opening a G-code file. If it works, restore all your Mach3 settings manually (without copying the old XMLs).
If that does not work, then a clean Windows reinstall is probably the next step.
In general, running CAD programs, browsers and other stuff on the same computer as Mach3 is not the best idea. Mach3, especially running through the parallel port, wants to have its own dedicated computer.