This service is essential for lots of things, including security in general and antivirus. If it’s missing on Vista or Windows 7, it was probably removed by an infection. A good first step is probably here:
http://kb.eset-la.com/esetkb/index?page=content&id=SOLN2861
Look for the “ESETSirefefCleaner tool”. If that doesn’t do it, try the steps here:
http://www.hageltech.com/blog/2012/02/07/base-filtering-engine-problems.html