Choose a program to open with appears

Hello,

Today while I was browsing the net, my anti-virus software told me that I had some trojan issues on my computer and it wanted me to quarintine it. I did, and after that, everytime I click on something like Microsoft or Internet Explorer, ect, it asks me to choose a program in which to open it with. It never asks me that before! And now, whenever I choose a program (like Internet Explorer with Internet Explorer) it keeps sending me in an endless loop of "You are sure you want to do this?" "Choose a program in which to open this!" "Are you sure you want to do this?" etc. What am I supposed to do? Everything else seems to be working fine, I just can't open anything!!!

Please help!