First you have to load the new key in the PIX. This is done by following an upgrade procedure (even if you load the same software). The PIX will prompt you with the question if you want to install a new key. Respond with yes and type in the new key. The PIX will support DES from now on.
The next step is to configure the PIX for dynamic IPSec. There are several examples on the Cisco website that explains the IPSec configuration in detail.
I would consider to replace your VPN V1.1 client with the newer Cisco Secure VPN client V3.0. It allows you to push configuration settings (such as WINS server, DNS server, IP address, etc) to the client.
Good luck.