ISA Cache is really meant for networks with hundreds of users, not a small network with a 2Mb connection.
The reason for this is that if you imagine a large network, people may all be accessing the BBC, or another news website at the same time. Caching is great for 20+ users accessing at the same time, but if you only have one or two, by the time you visit the site again, the content will most likely be out of date and need to be updated.
...As well as many websites with dynamic individual content (look at this site for example!). It just makes cache servers pretty much outdated... and if anything, may actually be slower.
This is just a friendly warning before you spend thousands on hardware and licensing needed for ISA.
That being said, if I have not convinced you, ISA is pretty easy to install.
The two easiest ways are as follows:
First - a machine with two network cards, have one facing the internet and one facing your internal network, you then use the ISA machine as a router, you can either set up the ISA machine as a proxy server on the clients or just install the ISA client to auto configure the machine.
Second - easier, use a machine with one network card, just on the network and again, use the client or set a proxy server. As far as I remember, there is no template for this scenario and whilst you can still use ISA for security (web filtering, logging etc.), you will not be using it as a firewall.