You assume someone has to purposefully attack you in order to have any chance of exploiting you. This is a mistake. Botnets of tens of thousands of computers aren't brought about because there are thousands of crackers working around the clock breaking into people's computers, they're caused by exploiting two things:
1) User ignorance
2) Software flaws
While you can learn to minimize the first one, all the experience and training in the world won't help you on the second unless you find a way to formally prove that all of the programs you use are unexploitable. Given that there are dozens of exploitable bugs found in everyday software each year I think it's fair to say that nobody's even close to that yet. Considering that many of these bugs require no user interaction whatsoever (drive-bys) I think it's arrogant to presume you're invulnerable to them.
The WMF exploit is a case in point, a bug that allowed arbitrary code execution was unfound and unpatched across half a dozen different versions of Windows. All you had to do was visit a website or receive an e-mail that contained a WMF image designed to exploit it. I don't think your intuition for security would help you in that situation.
Anyway, I'm getting completely off-topic. The point is that security conscious users should be running with the least user access they need to do what they need in everyday situations. As MSTCrow has pointed out, this can be a major problem for mIRC and I can only imagine that Vista reacts very badly to this situation with UAP enabled. It may not be a bug per-se but it's definitely something that needs to be changed for mIRC to continue running smoothly on modern OSes.
