An "addon" like NoNameScript *is* a script (it's even in the script's name), and a fairly large one at that. Unload it as was suggested and test this issue again.
Using scripts and having problems with mIRC is far less "random" than problems with a clean copy of mIRC- especially since NNS has been the cause of many similar CPU usage issues in the past.
Also, the Vista comments are not exactly qualified. Ping/pong events happen once every ~120 seconds. It would be impossible for one event every 2 minutes to account for 90% utilization no matter how slow GDI was. Secondly, not displaying the ping/pong emits no GDI events, so if you claim it's the same either way, that rules out GDI (or Ping/Pong) to begin with. Frankly, the only way GDI could pull mIRC to 90% is if you're in 50+ high volume channels with 10+ messages per second or something..
On that note: how many channels / networks to you have open?
