I have 2 5850s, and if I try to stop a miner to change settings or test a new mining program, my computer just crashes.

The settings I use for my dedicated-mining GPU is -v -w128 -f1, could there be an issue with the card?

I had the same problem with 2x5850 (Sapphire Extreme) under Windows 7,
i've installed Linux (Ubuntu) now i don't have any problem.

Some cards have trouble going from full load to zero load. Perhaps it's overly-aggressive in underclocking when it's done at full load, or could be related to the PSU -- I'm not sure. Does the same thing happen if you underclock your card and then go from full load to stop?

However, I don't think it's necessarily the 5850 model (brand may, however, be an issue) or Win7 since I'm running Win7 with 5850s and don't have this problem, though I did have this problem occasionally when I was running an overclocked 460, and I have no idea why (it had no stability problems outside of this). Wasn't just mining -- starting/stopping Furmark would do it on occasion, too.

cause RPC errors buy either unplugging your cable or disable LAN card, wait till 0Mhash/s on all miners and use ctrl+c to stop all not just one