2 isp's one router

Site A has two internet connections from two isp’s and one router. The router is   Cisco 1700 series connected to a PIX 515e. Site A has an MPLS connection from ISP 1 to Site B which also acts as it’s internet connection. The source of ISP1’s internet is at site B. Site A  has two ethernet interfaces. The Fast0 is connected to  the MPLS link from ISP 1 whiles ethernet0 is connected to ISP 2. ISP 2 connection is via VSAT on the serial interface. We would like to create a failover situation at site A so that if the MPLS link fails, the system should be able to switch to the VSAT
for internet at Site A.


The config below only works if the weight on the default route on the serial0 is manually increased to the same level as the default route on 10.20.20.9. and the gateway on the Domain controller acting as DHCP is changed to the internal IP of the firewall.
How can the router's config made to work automatically without any manual intervention
 





Building configuration...

Current configuration : 957 bytes
!
version 12.2
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname XXXX
!
enable secret 5 $1$eLc/$3HMpM7vIesR8cH2dyXJ0k/
enable password ****
!
ip subnet-zero
ip name-server isp1
ip name-server isp1
ip name-server isp2
!
!
!
!
interface Ethernet0
 ip address A.B.C.D 255.255.255.248
 full-duplex
!
interface FastEthernet0
 ip address 10.20.20.10 255.255.255.252 secondary
 ip address 172.16.16.2 255.255.240.0
 speed auto
!
interface Serial0
 description Link to Internet
 ip address E.F.G.H 255.255.255.254
 no keepalive
 no fair-queue
 ignore-dcd
 down-when-looped
 no cdp enable
!
ip classless
ip route 0.0.0.0 0.0.0.0 Serial0 2
ip route 0.0.0.0 0.0.0.0 10.20.20.9
no ip http server
!
!
snmp-server community public ****
!
line con 0
line aux 0
line vty 0 4
 password *******
 login
!
no scheduler allocate
end