This is one of the biggest pains a network admin has to deal with.
Microsoft has WSUS that is free and can be used to patch windows components. It is easy to use and has good reporting.
The downside is it only patches Microsoft stuff so things like Java, Adobe ... won't get patched
Trying to get a product that does it all means you will probably have to spend some money.
Take a look at this thread for more info: http://community.spiceworks.com/topic/170134-opensource-windows-patch-management-tool