If you are using xp, There is an option that causes xp to reboot on an error. Go to system properties (right click on My Computer on the desktop) click on the advanced tab, where you will find startup & recovery info, click on the settings button for it and in the new box that opens uncheck automatically reset.
After doing that, you should be able to see what the error is thats causing the crash and we may be able to help. Also ask a friend to keep an eye out and let you know what they see when you disconnect. (if its 10053, software causes connection abort, check out
this thread )
-if you are using any of the "speed" patches that are supposed to increase download speed, they have been known to cause BSOD (and be aware they can make some registry changes you'll have to undo as well)
-some script you are using may be in conflict with the version of mIRC you are using so it wouldnt hurt to try a clean copy of mIRC to see if the same thing happens
-if the error is DRIVER_IRQL_NOT_LESS_OR_EQUAL run a search here using that phrase, set it to all forums and all dates to read what others have found