That's not really true, Gary. I use a higher end PC running XP Pro with 16 GB of RAM and a fast processor and large HD, and I still find that preplanned hard shutdowns preclude accumulated errors on long runs. Microsoft's operating systems are not designed to stay powered up indefinitely and benefit from periodic hard shutdowns (by "hard shutdown" I mean turn it completely off, wait a minute, then turn it back on...not just do a reboot).
Like Joe Johnston I also do long runs of 20-plus hours spread over several days. As noted in several posts above, the key to perfect multi-day runs is to maintain machine position. To do that I use the S-key to stop the 'bot, then quit, exit SB3 and shut down the computer. I leave the control box powered up. THIS IS THE KEY IN MY EXPERIENCE. I don't re-zero the 'bot, move it to 0,0, or anything else. The next morning I power up, warm up, and use FG to restart the run. It works perfectly for me.
During the day I also reboot the computer at lunch. To do that I follow the "overnight" routine I just described, only I restart the computer after turning the computer completely off for one minute. In years past losing position between reboots was an irritant that I had to deal with, but since settling on my current technique I have never been disappointed.
Oh...one more thing. I never zero off the work piece, since I often carve its entire upper surface. Instead, I always zero off the waste board at a position that I write in my log book for each piece. That way, no matter what happens I can run C3 to get a consistent 0,0, then go to the exact same X,Y time after time to z-zero after tool changes. Also, ditch the 0.121 aluminum z-zero tool...its CoTE is large enough to produce noticeable marks in the work piece after zeroing just a few hours apart (as the day heats up or cools off). Instead, use a .25 or even .5 or 1" steel z-zero tool...it is much more thermally stable.