how to make vpn computer to browse local resources

guys need help...

my computer A is connected through VPN to my LAN at home (Computer B & C)
thorugh VPN i can ping from A to B
I can only access resources from A to B when i turn OFF firewall on B
On my LAN i can access resources from C to B without turning OFF firewall on B

So my question is if A is virtually connected to LAN why do i need to turn OFF firewall? by the way VPN ends on my LAN at my router.

is there any way i can do any changes in B's firewall to allow access from A(VPN computer)?

B(on LAN) windows 2003
A(VPN) xp pro
C(on LAN) xp pro

thanks...