Secured connections from ADSL

Hello Everybody,

I have been asked to establish a secured WAN based on one static IP (DSL connection at the head office) and many dynamic IPs (branches connected through ADSL to Internet).

At the head office I have already ZyWALL 70 which can handle many VPN tunnels, even from dynamic IPs.

I would like branches to route all Internet traffic and network traffic through a central router and wonder if this is possible at all?

For example:

1. I will connect branch computer to ADSL through regular USB ADSL modem.

1. Than will install Zyxel VPN software (which is more or less SoftRemote program -
   formatting link
   on this computer and configure the VPN tunnel to the head office (to the ZyWALL router)

2. Than - don't know what to setup more so all network and Internet traffic will go through VPN tunnel (incoming and outgoing)

Ad 3. - Because branch computers are often Win98 (there are some XP) I would not like them to be connected with Internet totally unsecured. So this is the reason I would like this connections to go both ways (incoming/outgoing) through a central router.

Is it possible without buying a special ADSL router?

I will appreciate any advice

With best regards

Mike