This is happening because the VPN connection becomes the default gateway for your PC. If you want to have all your traffic go out your ISP and only traffic to your work go thru the VPN ( the way it should be ) then disable to 'use this connection as the default gateway ' option on the pptp setup - or whatever vpn setup you are using. I don't know about the exact wording, but it should say something about using it as the default gateway... once you uncheck that and redial in it should work better. You can verify this by going to
http://ip.ChangeIP.com while dialed in to see if you get the correct IP.
Sam