Wireless DHCP clients disconnect Cisco VPN clients on same LAN?

I’m a bit baffled by this one. I have an office with a Linksys WRT54G wireless router. One desktop PC is connected via ethernet to the router, whom I’ve given a static IP to. This desktop is connected 95% of the time via Cisco VPN client to the nearby hospital. When connected, if another person in the office uses the wireless in their laptop (Sony Vaio VGN-TX850P, DHCP) to connect to the Internet, the VPN connection gets broken. I’m sorry to say I just don’t get it….any ideas?