For the Record:
After rooting around, I noticed that my standard mouse software allows the mapping of a "Precision Booster." When enabled, it allows me to switch instantly from whatever my current sensitivity is to a percentage of that sensitivity.
So, for example, when using my mouse for everyday work, I have it set at one level. But when gaming, I could crank up the sensitivity to max, called "pointer speed" in Microsoft's mouse driver software. With this button mapped to precision booster, when I click it in-game, the sensitivity drops to from 10 to 90 percent of its current settings. And this works just fine for sniping in Quake 4.
I just thought I ought to put this out here, for all of you who, like me, are financially... trying not to spend too much.
Try it.