Cisco PIX 501 trouble with VPN

Our one office uses a Cisco Pix 501 series firewall, for some reason though, it just will not let a VPN connection to be established. The DC on the network is running Server 2003 Standard. Im not sure where else to look since I covered just about everything I could possibly think to look at. I recieve an error 800 where it tells me that it is unable to establish a VPN connection and that the VPN server is unreachable, or that the security features may not be configured properly. If anyone could shine some light on this problem I would greatly appreciate it.