I have a user at my company who has a similar problem and here is the fix we found from another website. Hope it helps
XP Firewall - connection dropping after a few minutes
Those using the Cisco VPN Client, on a system running Windows XP, with the Firewall feature enabled, may experience timeout problems (your session may disconnect within 5 minutes) if the following type of traffic is not allowed to pass through the firewall: UDP port 500
Any customers using a home router or "NAT" (Network Address Translation) box* should not experience any problems whatsoever.
* Devices that are used to share Internet connections within the home - connecting to Cablemodem services (e.g. Cogeco) or High-Speed DSL services (e.g. Sympatico) - from manufacturers such as SMC, NetGear, Linksys, D-Link and Microsoft.
Changes required to avoid timeout issues:
Start->Control Panel->Windows Firewall Under the "General" Options Tab, ensure that the "Don't allow exceptions" is NOT checked). That is, you DO want to allow exceptions ...
Next, select the Exceptions Tab ...
Select "Add Port"
Specify a Name for this exception, in the "Name:" field (call it anything you like. Example: UDP500)
Specify 500 as your Port number.
Select UDP.
Select OK.
Select OK again, to close the Windows Firewall window
