hamachi is what you want (if by non static, you mean when users are out of the office, if they are in the office, then emo hug is bang on)
give it a go, it creates a vpn and you can set it to startup as a service, the networks are sort of like an msn contact list
awesome tool, but you do have to install it on each machine and do a tiny bit of configuring
after the vpn is up, you just use your normal remote software to do the rest