You can install the vSphere Client in two ways, from the vCenter Server media or by clicking a link on the ESX, ESXi, or vCenter Server Welcome screen. The installer on the vCenter Server media (.iso file or .zip file) is self-contained, including a full .NET installer in addition to the vSphere Client installer. The installer called through the Welcome screen includes a vSphere Client installer that makes a call to the Web to get .NET installer components.
If you do not have an Internet connection, the second vSphere Client installation method fails with Error 1603 unless you already have .NET 3.0 SP1 installed on your system.
To workaround the issue, establish an Internet connection before attempting the download, install the vSphere Client from the vCenter Server media, or install .NET 3.5 SP1 before clicking the link on the Welcome screen.