This is yet to be officially announced by Cisco or Microsoft, but reports from the Hyve testing lab is that there is a compatibility issue with Cisco VPN Client and Windows 10.
Users are having to uninstall their Cisco VPN before being able to upgrade to the Windows 10 OS. If there is a Cisco VPN Client installed on your machine, it will disable your network adapter when you upgrade.
We are advising our clients to not upgrade their systems to Windows 10 to avoid the issue with the VPN. We will wait to see if either Cisco or Microsoft update with a solution to this.