Question : Why can't I connect to VPN using IPSec security?

Question:
Attempting to connect to a client site via VPN.  Client IT person gave me approriate IP address of host, IPsec "pre-shared key for authentication", user name & password.  Other instructions were to use TCPIC, and uncheck use default gateway.
REMOTE COMPUTER 1 (XP SP2, LINKSYS ROUTER, CABLE MODEM), connects to the host without any issues once I made the SP2 Resistry hack (http://support.microsoft.com/kb/885407/en-us).
REMOTE COMPUTER 2 (XP SP3, LINKSYS ROUTER, COMCAST MODEM), Connecting to VPN... window appears, then hangs for a couple of minutes returning Error connecting to VPN (error 800 when checking more info).

Computer 1 and Computer 2 are in different geographical locations.

I've turned off all firewalls on computer 2, and Googled it to death.  I also made the SP2 registry hack.  I can connect to another VPN that does not use IPsec.  The IT person of my client tells me the above information is all he typically provides his vendors.  Once they make the reg hack, there are no issues.
I've also checked both linksys routers and the setting are identical.  All appropriate ports are open.

Do I need to open a port on Computer 2?  I've cross checked the setting from computer 1 to 2 several times, and they are identical.

Any steps to try would be appreciated.

Answer : Why can't I connect to VPN using IPSec security?

I would assume the site to which you are connecting uses certificates. It is possible the existing root certificate information (on your PC) had reached the expiry date requiring the root certificate update to make it current and acceptable.