I think I'm infected but I'm not sure. Help?

Hi so recently my idiotic brother (sorry) who is obsessed with Maplestory had recently decided that it'd be smart to download some hacks onto my laptop… Sadly spoken, it turns out that in the end he got hacked and I believe that my computer might be keylogged/infected. I had asked him some questions and this was the story so far. Apparently he had downloaded a hack that was already scanned and appeared to have a few viruses but due to "other comments claiming that it works," he ignored the signs and decided to act on his own accord. While upon downloading the file, he claims that IT DIDN'T WORK so he HAD deleted it and placed it into the trash bin. He says he cleared out the trashbin as well as preformed a disk cleanup and a disk defragment on my computer not long afterwards. It wasn't until 10 minutes after downloading/ deleteing the file did he decide to go back onto Maple just to find out that someone was already signed onto his account. He says he hasn't given out his information at all and now, most of his stuff is gone. He believes that he was indeed keylogged by the wesbite or something (idont know his weird language) and states that he had never given out his info and that it WASNT UNTIL after downloading the hack, did the problem finally occur. I think my computer is now infested and that my brother's accounts were keylogged or something and I would like to know if there is anyway to remove these malware/viruses. I don't have any actual anti-spyware programs except for the Avast!On-Access scanner7. I use to have Norton but I had uninstalled it due to it slowing down my computer. I can't seem to find any answer to my problem and this is my first time on this website. I was recently introduced to this website because I had told my friend about my problem and well.. now here I am :). What really sucks most about this whole situation is that my brother… is freaking out about having his other account hacked and has decided to run Maplestory all throughout the next day on my laptop until he has found a solution for this problem… As much as I'd like to just shut down my computer :\ I do feel bad for him and I myself would also like to get rid of these stupid viruses as soon as possible. Please help me, I've also recently noticed that my computer has been acting slower than usual.

I am using Windows Vista by the way Also I'm Not sure if it might be of any use/help but I was using FireFox so maybe theyve managed to acquire the information for my brothers account through the cookies/passwords w.e saved onto my firefox :\ I've currently deleted all the history/cookies/etcs. on firefox and just switched to Internet Explorer for the moment.