Try enabling logging for both the successful and failed attempt
http://www.cisco.com/en/US/docs/security/vpn_client/cisco_vpn_client/vpn_client46/win/user/guide/vc7.html#wp1332321
Set the logging options to high for everything
http://www.cisco.com/en/US/docs/security/vpn_client/cisco_vpn_client/vpn_client46/win/user/guide/vc7.html#wp1313800
Also, get the output of a netstat -r and ipconfig /all for both working and not working. Should compare the results to determine if there are any deltas between what works and what does not.
Also, see the following restrictions
Unsupported Features
The Cisco VPN Client for Windows 7 and Vista does not support the following features:
•Upgrade from Windows XP (clean OS installation required).
•Start Before Logon
•Integrated Firewall
•InstallShield
•64-bit support
For x-64 support, customers should explore with their Cisco sales representative the use of the next-generation Cisco AnyConnect VPN Client.
•AutoUpdate
•Translated Online Help - Provided only in English
•The VPN Client on Windows 7 does not support WWAN devices (also called wireless data cards).
Windows 7 and Vista Window Auto-tuning Feature Might Cause Network Timeout Problems
Windows 7 and Vista support a feature called "Receive Window Auto-Tuning" that continually adjusts the receive Windows size, based upon the changing network conditions.
Some people reported that auto-tuning causes network timeout problems with some applications and routers. If you have experienced such problems, you can turn it off using the following procedure:
Step 1 Open an elevated command prompt.
Step 2 Enter the following command to disable auto-tuning:
netsh interface tcp set global autotuninglevel=disabled
If this does not fix the problem, you can turn it back on, as follows:
Step 1 Open up an elevated command prompt.
Step 2 Enter the following command to enable auto-tuning netsh interface tcp set global autotuninglevel=normal
To view the states of the TCP global parameters, use the following command:
netsh interface tcp show global