Ok, two things I would try, first is redownload Mach just in case you got a corrupt install, the other is go to device manager and see if there is a driver there, (Called Pulsing Engine if I recall), if there is delete it and then install Mach again.
I am presuming that it is a 32bit windows version you are running as you said you had previously had it running fine, but thought I would just mention it just in case you have just upgraded the computer as the Mach driver will not work with 64 bit operating systems.
Hood