Nigel, if you search the archives here there has been quite a lot of discussion about the packaged controllers - initiated especially by the late John Stevenson, rightly highly respected here, who fitted quite a few of them in upgrading older Denford CNC mills. He rated them highly. There are a few China based suppliers.
The PC's housekeeping issues are avoided by using a separate motion controller that does all the time-critical stuff, with USB or (better) Ethernet link to the PC. Having said that I have never seen a problem with Mach 3 / Win XP which just takes over the PC at a lower level than DOS.