I also have winXP pro SP2 and have no such problem... I don't think that is the reason what is causing the problem but it's probably something else.
try this: copy the mIRC.exe to a random different directory. create a clean text file called mirc.ini in the same directory. now double click the mirc.exe
does it still not start?
now try repeating this without any AV or anti-anything programs running.
