Explorer.exe does'nt start automatically!

Hello,
i had a virus attack (mabezat) but after scanning the comp for about a week and using various virus removal tools my comp was free from viruses but the explorer.exe file was missing i tried following the steps of running a new task open explorer.exe but it was'nt found so finally i copied it from my friend's comp and it is workin fine but then i have to open the explorer.exe file each time i start the comp it does'nt happen automatically.. can anybody help me with this? thanks in advance:)