Thanks to both previous answer-suppliers. My own solution is the following:
By searching in the C:\Program Files\TideSDK Developer folder and sub-folders for the phrase "no Desktop SDK versions" I found where the alert comes from -- Line 1666 in C:\Program Files\TideSDK Developer\Resources\perspectives\projects\js\projects.js. In the code, I noticed that the alert results from a empty result from a call to Ti.Project.getSDKVersions(). By perusing the TideSDK API docs, I determined that getSDKVersions() relies on a call to Ti.API.getComponentSearchPaths(). When I inserted alert (Ti.API.getComponentSearchPaths()); at Line 1665, the result was two PATHS -- One was C:\Documents and Settings[my current user]\Application Data\TideSDK and the other was
C:\Documents and Settings\All Users.WINDOWS\Application Data\TideSDK
NOTE: All Users.WINDOWS, rather than All Users. So, I copied the TideSDK folder to the above location (and removed the Hidden attribute, as suggested above) and now Developer finds TideSDK and all is well.
I don't know if the above solution applies to ALL TideSDK installations on Windows XP or not, but if anyone has been receiving the "You are importing a desktop project, but no Desktop SDK versions exist on your system" alert in TideSDK Developer on XP, I suggest giving this a try.
Jonathan (neologis)