Well if you've used all the information here I doubt it's a problem with mIRC itself. Perhaps try another IRC client and see if it works with that.
You might try contacting your ISP and see if they have made any changes since you started experiencing this issue. Although I also doubt it's a Windows problem try using System Restore (for XP/ME) and set it to before this problem started and see if that helps. You could also try Microsoft's Support site
I think out of all the information, your ISP is the most likely issue.
Just an FYI, you do have 2 hours to edit your post, so if you forgot to include something, you can go back to add to it rather than posting twice