Make sure that your operating system supports the vSphere Client.
For the most current, complete list of supported operating systems for the vSphere Client, see Supported host operating systems for vSphere Client (Windows) installation.
The vSphere Client requires the Microsoft .NET 3.5 SP1 Framework. If it is not installed on your system, the vSphere Client installer installs it. The .NET 3.5 SP1 installation might require Internet connectivity to download more files.