You might be able to do with with an app that generates mock locations (usually used for testing purposes). No idea if someone has actually written such an app, though, and it’s conditional on HTC leaving the “Allow Mock Locations” option in the settings page.
Odd. I find my Desire S’s GPS to be far more accurate than it really should be for a mobile… A lot of my riding’s under trees and it still seems to be accurate – within 10m most of the time, certainly rarely more than 50m
That said, some apps are better than others at filtering out bad results. I use everytrail pro and find it pretty good.
I would seriously consider whether there’s a problem with your phone…
(and thinking about it, this is my second Desire S – the first was similarly accurate)
edit – thinking about it more and having a quick look at my logged everytrail routes, it’s typically accurate to about 5m – the tracks certainly follow the actual trails on the map.