What do you mean by "it" won't let you close mIRC? What is "it"? It sounds like mIRC might be frozen, which would most likely be caused by a faulty script.
Did you have scripts installed in 6.35? Make sure you install a completly clean install of 7.1. This means backing up all of your settings (in the %APPDATA%\mIRC folder), deleting those settings (not the backup, of course) and installing 7.1 to a new directory (if 6.35 is in Program Files\mIRC make sure you choose another location).
