Hi,
first, are you running any wind0ws server at your work? if so you can set it up there depending on your router...
Or, the other way of doing it is with 2 old (PI >) boxes you can load with linux based ipcop or smoothwall, with 1 @ home and the other @ the office. This is the best method, very secure and makes a good firewall. VPN takes precise configuration to work though, it's all online at
http://www.ipcop.org or
http://www.smoothwall.org
In both cases you'll need to get a free dynamic DNS account (
http://www.dyndns.org), that will take care of the changing IP address. if you have a decent router that supports dyndns you can use that to update, or ipcop and smoothwall both have built-in support for dynamic dns updates.