The fact you said, other programs also & you scaned and deleted all viruses inferes that there were some viruses and this problem exists outside of mirc. Well not all virus remove cleanly, one may have been removed but has destablized your entier OS causeing programs to fail.
While I dont quote this as a fixall, i have often found that simply installing the OS ontop of itsself, well fix alot of these little things, since its an install ONTOP of itself (not a REINSTALL) your programs etc well remain. I also wouldnt be suprised if there is still some spyware onboard, try MS antispyware app as well, the damn thing misses thinsg others get but find things they miss. Tipical MS software (even tho they just brought it from someone else).