Which version of VPN Client were you trying to install?
Further to your query, you can even try the following -
1. Uninstall Cisco VPN client that you may have installed and reboot your system
2. Install Cisco DNEupdate.
3. Install the Cisco VPN Client Cisco VPN Client 5.0.4.0300 or newer version
If the above does not work, repeat steps 1 and 2.
3. Make backup and Delete the files ndis.sys from c:\windows\system32\drivers and ndis.sys.mui from c:\windows\system32\drivers\en-us.
if you are not able to delete it, change the permission to 'Everyone'
4. Install Cisco VPN Client
5. Insert your Windows 7 installation CD and restart into the Windows 7 install when prompted.
6. Select language, time, currency and click 'Next'.
7. Click 'Repair your computer' at the bottom.
8. Click 'Next' to start the repairing the OS.
Windows 7 will repair itself and automatically reboot.