I found quite a neat little trick, whether or not the following technical processes are by design I dont know but I assure you this has worked for me loads of times.
If you see Sophos Anti Virus with a red shield in it, first thing I used to do is rename the Cache folder in Program Files and then click update. Job done, it updates we can all go home with a smile on our faces.
What I noticed recently was that this didnt fix the problem at all, it generally made a bit more of a mess.
So, if you find that the Cache folder has an update in there (Usually 68Mb+) Navigate to: C:\Program Files\Sophos\AutoUpdate\cache\savxp and click Install Sophos Anti Virus, this will run you through a quick WinInstaller and then update the program.
You will probably have to reboot at this point, you can choose to reboot, usually I just log off ;) (As im sure most IT pro's do)
Finally just re-enter the update credentials into the Sophos program and re-check for updates.
Works like a charm
Did I solve your problem? Buy me a virtual beer by clicking on a Google ad :). Thanks!