How do I build a simple but secure VPN?

As a school project, I have been tasked to build a small, simple, and secure VPN to allow traveling engineers to access resources on the home office server. I have at my disposal a Windows 2003 R2 server, Windows XP and Windows Vista workstations, a LinkSys WRT300 wireless router, and a Thompson 546 ADSL modem (the ADSL service is provided by my employer, behind firewall and proxy servers).

Where can I get information, procedures, or other project notes to develop this small scale VPN? Any and all assistance is greatly appreciated!