fss, a whole host of idea's runs though my head, but mIRC isn't one of them

You plug in dsl and you're 'on line' ? How does your password get validated with your DSL carrier ? Something's missing from the equation here. You have a nic card in your pc, and you have a dsl modem, but no software for them to interface ??
Can you connect to mIRC, join a network, with a nickname not known to anyone and just sit but not be on a channel... basically, join so 'NO ONE' knows your there... let it sit and see if the disconnect still happens. Or, better yet, keep mIRC closed, open up a web page that auto refreshes, and just leave it, see if your dsl disconnects you
mIRC isn't 'killing' your dsl connection, is what Im getting at. In order to figure out exactly what is happening, you need to isolate each step in your connection process. I don't run XP, I won't touch XP with a 40' pole, so if the problem is XP related, I"m afraid someone else will have to offer suggestions. However, I do run DSL, I have had extensive ... situations with mine, so that aspect I can continue to offer you help, but ultimately a) Your DSL is dropping you. b) Something outside your system is 'disconnecting you' because it truly shouldn't be (and really couldn't be) mIRC 'doing' this. Let's have you do the above steps and keep digging at this. Maybe others have some suggestions as well.