Adicione as seguintes linhas no: /etc/squid/squid.conf

refresh_pattern windowsupdate.com/.*\.(cab|exe|dll|msi) 10080 100% 43200 reload-into-ims
refresh_pattern download.microsoft.com/.*\.(cab|exe|dll|msi) 10080 100% 43200 reload-into-ims
refresh_pattern www.microsoft.com/.*\.(cab|exe|dll|msi) 10080 100% 43200 reload-into-ims
refresh_pattern au.download.windowsupdate.com/.*\.(cab|exe|dll|msi) 4320 100% 43200 reload-into-ims

depois basta dar um reload no squid com o comando: /etc/init.d/squid reload