Two words: PowerPro, again. I know, it sounds like I'm a freakin' PR manager for it--which would be unusual, since it's freeware.
PowerPro can show a Start menu at the mouse position when you left-click, right-click, middle-click, whatever, on the desktop or anywhere else you choose. Another alternative is to show the Start menu when you simply "bump" the mouse against an edge of the screen.
I keep recommending this program because it really can do almost anything, in terms of interacting with the computer. But many people find it very complicated to set up, which is what keeps it from becoming a hit. If anyone is interested I can post a single-purpose PowerPro config file for the Start menu thing or any other thing, when I have spare time. Then someone can just overwrite the default config with the single-purpose one and instantly have the feature they want, with no effort. With a simple config, PowerPro only takes up a few hundred KB in memory.