Assuming that both routers (A and B) each have a connection to the ISP (with a public IP Address on each of them) ...
In that case, you would want to use hsrp on the inside interfaces of both routers, and use that address as the PCs default gateway.
Because each router would have a different IP address on the internet side, you WOULD drop any tcp connections from PCs out to the internet ...
but that is generally not very painful.
Rob