mIRC is not at all exploit-proof. I wouldn't run it from an Administrator-class account. But then again, I don't run ANYTHING from an Administrator-class account if I don't have to. Use Limited where possible, is the idea I keep pimping on people. So much safer.
What I would do here, if it were me? is to back up what matters to me onto CDs or DVDs or another hard drive, then blow away the Windows installation and set it up right this time. Make your first user account called
Admin and then make a separate one for yourself, and also one called Visitors. Use
Admin to get everything set up, and password-protect it. Make your account Limited, password-protect it, and arm the screensaver to lock at 15 minutes and to show the Welcome screen on resume. Make the Visitors account non-password-protected so your pals can use that one if they come around.
This would (1) keep your pals out of your stuff, since the Visitor account can't get into your account, and (2) ensure that both you and they are "wearing your seatbelt" with Limited accounts. If you really can't stand to use a Limited account, make your own account a Computer Administrator, but use the Limited-class account named Visitor for anything "risky."
/me sees bmetzger slowly backing away and feeling for the door, what have I stumbled into here...
Anyway, /me has a bunch of security suggestions for people who are setting up Windows
here.

Since you have a router, one other suggestion: lock down all ports that you do not actually need.
20 & 21 for FTP
25 for SMTP email if you use it
53 for DNS
80 for standard Web
110 for POP3 email if you use it
143 for IMAP email if you use it
443 for HTTPS Web (secure sites)
and whatever port mIRC uses, I should know it but I forgot

A Google for mIRC Port Firewall will get it for ya
Locking the unneeded ports keeps stuff
in, not just out. If your computer gets infected with a Downloader or SDbot or Gaobot or Trojan that is designed to "call home" on a random high-numbered port, this will keep it contained. Closing the IRC ports would be highly desirable, they're a prime target, but you can't do that if you want to use them
Anyway, hope that helps a bit
