-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. You should then be able to see what error msg you get.
-if you are using a fileserving addon, have you checked to see if it still crashes when not using it? As in with a clean copy of mIRC without that script or by disabling it.
-some speed patches/booster things are known to cause BSOD
-your provider could be using interference techniques or have limits on what you can send
