I NEED PC HELP (Possible Virus)!!!!!?

I installed some software and seem to be working fine. 20mins later i tried to download some updates from microsoft but i couldn't.(page wouldn't load). Then wierd things happened. 1-I tried to use the SEARCH option under start but I can't. (says its missing a file, the companion dog comes up but no background behind him.) 2-I can't acces the restore system, just a blank white screen comes up but nothing else. 3- i can't use my original antivirus (yahoo suit), when i try i get a blank screen and a pop up thaat says internet explorer runing bad scripts, do u want to continue. The only thing i can do is close it but i must close it like 20 times. 4- i can't use online virus checks b/c the pages won't load, but all others will. I already used many anti viruses lie AVG, macafee,norton and kapersky free trial. they found 14 trojan horses(trojan clicker.HTML Agent.a, trojan downloader .win32.small.ddp) , i deleted them all but still the problems continue. what can i do.!! p.s. before i installed the software everything worked fine, could the trojans have messed w/t my system and if so how do i fix it. PLZ HELP!!!!

I NEED PC HELP (Possible Virus)!!!!!?vista

The trojans have definitely messed up you system. The fastest and easiest way out (if you are the only one using your PC) is to format your hard-disk %26 reinstall your OS. Before that backup all your important files on an external media like CD/DVD/Flash Drive. Formatting will erase all data on your hard-disk.