reply to Lea Massiot
Re: VPN between two Cisco 887VA devices
Since the VPN does not terminate at the PC, VPN tunnel is transparent to PC interconnectivity. Therefore turning off PC's Windows firewall does not necessary make the VPN tunnel establishment work rather it allowed some TCP or UDP traffic between the two PC to flow through.
Lea Massiot said
I've turned off the Windows firewalls on both "PC 1" and "PC 2" and now "PC 1" can access "PC 2"'s shares and and vice versa. It is not a solution but I guess it is an indication that the VPN tunnel is actually "doing" its job, isn't it?
If you like, you could do the following test. Disable VPN between the two routers and have the two PC interconnect directly (read: via clear text without the VPN encrypting tunnel) while keeping the Windows firewall on both PC to be on. When the two PC are unable to connect, then it proves that VPN has nothing to do with the interconnection issue.