Complex sentence: Previously a games developer at Dynamix and a developer on the Windows NT operating system at Microsoft, Harrington founded Valve in 1996 with Gabe Newell, another former Microsoft employee: he and Newell privately funded Valve through the development of "Half-Life".
Split into simpler sentences: Previously a games developer at Dynamix and a developer on the Windows NT operating system at Microsoft, Harrington founded Valve in 1996 with Gabe Newell, another former Microsoft employee. He and Newell privately funded Valve through the development of "Half-Life".