Dreamfleet do a half-decent implementation of the 737-400 for Microsoft Flight Simulator.
I'm not a 737 pilot, just a modular ATPL student so I can't say for sure how true to life it is, but I have used it a bit in conjunction with the Bristol Groundschool ATPL course.
It's not exactly fixed-base sim quality, but it has a pretty good implementation of the FMC and EFIS displays. The aircraft systems aren't all that well simulated however, but for what I use it for it's fine.
Their website is
here .
Regards,
Gordon.