fill missing svohost.exe

Recently my system with winxp got affected by some trojan , back door virus . so I got the system shutdown message within 60 second. After installing norton2005 I got relexation from this problem . But after this I am getting a file missing error at startup. the file is “svohost.exe”. Windows cannot locate the file.But it shows the location C:\windows\system32\svohost.exe. I donot find any file there.How do I extract the file in winxp . is there any sfc command like win98se.
why the file is important .