"It's impossible to say how computing would have changed"
We can get some ideas how things would be different by looking at the browser market. In particular we can compare the IE6 period, when Gates' monopoly was at its peak, to how it is now that Mozilla, Google, Opera and Apple has revived competition:
-Focus on lock-in
After Competition was Revived
-A wealth of innovative features introduced
-Fair competition based on the quality of the product rather than lock-in
-Improvements in speed, security and stability
-Browsers generally following standards making things easier for developers
We could then look at how the OS and Office market is now and, based on the browser market, make some guesses at what the world would have been like without Gates:
OS and Office Suite Market with Gates
-Focus on lock in
OS and Office Suite Market without Gates
Executive summary: a shit load better
There's no denying that Gates is a great business man; he has ruthlessly strangled completion, locked in users and abused his OS monopoly to expand into other markets. This strategy has made Microsoft biggest and most successful company in the IT industry with obscene profits and massive influence, so it's hard not to be impressed by his business skills. However, everything he has contributed to IT has been negative and if he never existed we would likely have vastly superior software than we have under the Microsoft monopoly. Personally I would rank Gates as the single man who has done most damage to the software and IT industry.