You can configure your ISA Server to work as a VPN Server too. You don't need any other software.
http://support.microsoft.com/kb/837355
If your office network consists of a domain then you could easily setup VPN connections on your clients via group policy & powershell scripts. If not, you could just give them the script to run.
http://technet.microsoft.com/en-us/library/ee431700(v=ws.10).aspx