Newell said that Windows 8, the new Microsoft operating system, was "unusable".
He told The Verge that the new operating system was like this giant sadness that had hurt everybody in the PC business.
Rather than everybody being excited to go buy a new PC, buying new software to run on it, they have had a 20-plus per cent decline in PC sales, he added.
According to the Telegraph, before founding Valve, in 1996, Newell spent 13 years working on Windows for Microsoft.
Last year, Microsoft announced that it had sold 40 million Windows 8 licenses in the first month after launch, an improvement on Windows 7, which sold 60 million licenses in its first two months.
Microsoft recently said sales of Windows 8 had reached 60 million.