Spyware, malware &/or viruses would be the first things I'd be confirming aren't causing a problem.
Once you've confirmed that your system is clean, the next thing I'd check are your system resources..how many processes are running? how much ram do you have? what size hard drive? Is your hard drive fragmented?
Any one or combination of those things can cause any program to run slow.
If, after you've eliminated all of those possibilities, try booting into safe mode and run mIRC and see if it's still slow.
Although, if it's still slow at that point, then I'm out of ideas.
