Need VPN advice

Hi,
I am very new to VPNs and have questions about implementing one.
We have 5 offices and some home users. Some of our offices are using a dial up router/internet appliance. This router has its own firewall.
A couple of our offices are using a T1 router which both have firewalls as well (different companies though).
Then we have our office people who just use a regual dial up ISP.
I want to be able to implement a WAN via using a VPN. So I want to link up our offices. Also it would be nice if the home users could dial into thier ISP and then access files on our servers.
I do not really want to mess with the configurations of the firewall on our routers.
I read that I can run a VPN parallel to the firewalls which means this would require no changes to the firewall configurations themselves.
IS this true?
I read that there is Software only VPNs (not NT).
Can I then install VPN software on 1 computer in each office which will then create a VPN with all offices and not have to do anything with the routers' firewalls?
If so other than a static IP address at each office what else would be required?
Do you know of what would be the best VPN software would be the best for this as far as ease of configuration, security, and functionality?
Any insight on this would be greatly appreciated.

Thank you very much in advance,
Dani

Short answer: Take a look at Checkpoint's VPN solution.

It's what we're using here at work.