cant get update ubuntu 12.04

can't update 12.04 lts. I am new to Ubuntu so tried the answer to question 118279, and as with the upgrage manager I get ' E: the package index files are currupted.No filename: field for package Ubuntu system-service'. Have reported the problem after the failure. This is the first time any sort of error has occured through the updating system. Have looked up a number of errors of this kind and and the nearest looks like bug 801120, unfortunately the discussion becomes too technical. I hoped to fix this by using the info available rather than running for cover. I need some info to replace the missing file name other than that outlined in in answering question 118279