A few more shot in the dark suggestions...
What version of mach3 are you using... The recommended one is ....0.62... To replace your existing one id suggest copying your profile XML file to your desktop, deleting the mach3 folder and installing 0.62, then copying your XML file back into the mach3 folder and everything should be back as it was fingers crossed without the problem...
0.62 is available in the m3 ftp site
Have you tried another PC (laptop) or anything... I was going to look into printer ports and the BIOS as I thought that at one time you could select different printer port types in the BIOS... Not sure what mach3 prefers...
Another PC should split the system in two... Same problem... Then its a bob onwards hardware fault... Problem gone it's a PC fault...
Rob