I looked into this a little more. By using the unicode version of the Windows file creation API (which indicates that filenames up to 32000 characters in length can be created) I was able to make mIRC create filenames longer than 260 characters under Windows XP - but only slightly longer. I was unable to view/delete/rename these files in Windows explorer. I had to go into the DOS prompt to remove them. It looks like XP has very limited practical support for filenames longer than 260 characters. I'll be changing the file routines in mIRC to truncate filenames (including path) at 260 characters.