Cannot Download Critical Updates

I have several Windows XP computers that hang when trying to download updates from the windows updates site. It will scan the computer, and find the updates required, and when i select ‘install now’ it brings up the EULA. When I accept, it brings upthe downloading dialog box, then just hangs. No events recorded of either the Client or server. The client connects to the internet thru an ISA server, but I dont think it is blocking anything. All other internet browsing works fine, as do Symantec ‘Live Updates’ Any Ideas?