There were hundreds of changes between v7.25 and v7.27, so it would be difficult to know exactly what particular change may be the cause. That said, I usually only test mIRC on fully updated versions of Windows, as service packs fix a whole range of Windows bugs and issues. It would be difficult to cater for all of the potential issues, side-effects and subtle bugs that older versions of Windows have. I am not aware of any feature in mIRC that caters for such issues. My advice to users is that they should always update Windows as not doing so leaves them exposed to a whole raft of security issues. I currently only have Windows XP SP3 installed, so I am unable to test under anything older than that unfortunately.
