Microsoft Update keeps systems up to date. whenever you are updating through http://update.microsoft.com it is costing your bandwidth. Everybody does it repeatedly. So a clever way to manage this issue is to add a cache rule in Microsoft ISA server. It will cache necessary Microsoft update. whenever a request for Microsoft update pop up it will check ISA cache. this will save bandwidth as a result save money too. Here are the screen shots to help you with the cache rule.
Open ISA Management console>View>check task pan
Toolbox>Network Objects>Domain name Set>New Domain Name Set
Type Microsoft Update Domain Name Set as Name and Add all the update sites mentioned here. Apply>ok.
On the left hand side pan, expand ISA array>expand configuration>right click on cache>new>Cache rule
As you have added a cache rule, it might take a bit disk space. so change cache drive from system partition to another. In my case, I set up D:\ drive for urlcache. To do that right click on cahce>cache drives>Set D:\ drive or different in your case.