Restrictive Internet Access in Office?

We have a simple TCP/IP LAN in our office with each machine having their own TCP/IP address. We also have a DSL connection which is shared between all users (roughly 10-12 PC's). We would like to restrict internet access to some of these PC's but need all to have email access (via outlook). Is there an inexpensive (or preferrably free) method to do so? I have heard of hardware firewalls but these are quite expensive.

The best solution would be to block the internet ports on the pcs you dont want to use the net. This would allow mail traffic since it uses a different port. Port 80, 81 would be Http, https traffic.

Port for VPN connection: 1723

Ports for Outlook / Exchange communication: 135, 1025, 1026, 1225, 1226, 1228

Allow these and you should have good mail and no internet to the pcs of your choice :)

this is done from the router if i forgot to mention it.

The lan admin can limit domain access. We restrict access to youtube, myspace, etc.. Mainly high bandwith sites.

If your user are using Internet Explorer to go on-line You can do that :here is how you do it :

open Internet Explorer click on tools >>>click on Internet Option (Option)>>Click on content tab>>>on Rating click on Setting>>
click on approved site>>type some site then click any site name and select always then go ahead and make a password ,now go ahead and test it ,Now you can not go anywhere in Internet UNLESS you have the password ,now go back and do the same thing go to content and add your EMAIL server site address the put no password .
before you let this to other PCs practice with it few times You will get good at it ,don't forget users can not check content it will ask for password.