Newell, whose company is known for the Half-Life game series and runs the Steam games store, said Windows 8, the new Microsoft operating system, was "unusable".
"Windows 8 was like this giant sadness. It just hurts everybody in the PC business," Newell told The Verge
. "Rather than everybody being all excited to go buy a new PC, buying new software to run on it, we’ve had a 20-plus per cent decline in PC sales."
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. Earlier this week, Microsoft said sales of Windows 8 had reached 60 million.
Valve is working on a Steam Box games console that will bring its games into the living room but also act as a networked games device for the whole home.