What proxy server software is recommended for a small office?

I'm running a Windows 2003 Server R2 server with two network cards and 6 users XP. The server does all: File, DHCP server, domain controller, and routing. Instant messaging server looking for a proxy server software free to let me at least: Restricting certain websites (or allow only the necessary!), View record web navigation for all users, restricting some Programs such as Messenger, Skype, etc. and some basic characteristics. What do you recommend I use?

you can edit the host file on the PC you want to ban sites

Inviziweb Stage2 Windows XP

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