I'm afraid I cannot help you further as I am not an Check Point VPN-1 expert. I can tell you, however, that it won't be as simple as firing up the XP VPN client and pointing it at the VPN-1 server. You will have to examine the requirements of the connection to see if you can set the Windows XP the same. You should realize however, that the Windows XP IPsec VPN client automatically uses L2TP for the tunnel -- there is no configurable IPsec policy present. You can, however, create one and possibly match the Checkpoint requirements.
Sounds like a lot of testing will be in order here, and you may want to weigh the benefits of using the Checkpoint client and having a solution up and running quickly against the benefit of doing without it.
This was first published in August 2002