Do a search for update.exe, it might tell you where the program belongs to. It might be just a program you installed that f*cks up your system. It doesn't have to be a trojan.
If you find the update process belongs to a certain program you installed, uninstall it, and see it the problem still occurs. If so, then it probably a trojan of some kind. If not, reinstall the problem and if the problem return contact the company that made that program.
Hope this helps too
