if you're disconnecting due to software abort errors there are tons of possible causes for this-- and its been reported on every version of Windows, as RusselB said, so it's highly unlikely to be an incompatibility problem. Generally it's a router issue. I'm not sure how you're testing XChat, so it's hard to conclude "xchat works therefore it's mIRC's fault".

- argv[0] on EFnet #mIRC
- "Life is a pointer to an integer without a cast"