It sounds to me like mIRC is only freezing when initiating TCP connections, which sounds to me like something is blocking mIRC from getting a socket descriptor.
The only things I know that can cause that is a firewall/AV software, or a virus. Given that you've asserted your firewall/av are off, perhaps you should turn the AV back on and check for a virus. .. though it might be wise to double check that your AV/firewall are actually off. Keep in mind your AV might have its own firewall, and that there is also a Windows firewall to deal with.
Also of possibility: mIRC doesn't have privileges to initiate a connection, which would be more of a group policy guest account type deal.
