Updating Apps

When using my vista machine at home I do not have any problems with my portable app platform. When I use it at a library using Windows 7 I have trouble updating any of my apps. I get a message to update but on attempting the updates I get this error message "The download copy of [app name] is not valid and can not be installed. this could be due to an incomplete download or other network issue. Please try running the update again when complete". I just tried to update the portableApp.com platform on the Windows 7 machine and got the same error message. It has Sophos anti-virus, using windows firewall. Whenever I get this message, I can go home and plug it into my vista machine and update without a problem so it aint a massive problem, but it would be nice to know what might be causing it and if I can rectify the problem.
Many thanks.