In my opinion the best version to install on Windows XP or Vista is definitely Sidekick98 and the SK98 patch, although you can install that version and the Sidekick97/Internet Sidekick versions on the same computer, but you cannot run them at the same time. They must all be installed in a folder called Starfish on the root of the C:\ Drive (especially in Vista / Windows7 32 and 64 bit). What I mean by that is that within that starfish folder you should have 1 folder called Sidekick98 and another called Internet Sidekick, these would be created when you installed the programs in the C:\Starfish folder but you have to specify this during the install process. The reason for doing that in Vista and later versions of Windows is that they will not allow any changes in the C:\Program Files Folder which happens quite often whilst Sidekick is running (creating temp files of Cardfiles and Calendars etc)and again when you open sidekick or if Sidekick crashes.
The Sidekick99 Calendar is not backwards compatible with ANY of the other versions though you can open it in Sidekick98 but all ToDo's entered without due dates end up on the same day and all have a few irrelevant characters at the beginning of the line, but Sidekick99 has had a SERIOUS problem after Jan 1st 2006 in that you cannot add todos.
Sidekick98 has all that Sidekick99 has and more and is the Program I choose!
Sidekick97 (Internet Sidekick) has the best Export options (Contact log included as cardfield should you ever desire to change to another program) + File Synchronising and as I said before, you can install both on the same computer with the same cardfiles and calendars open in each, but you cannot and should not try to run them both at the same time.(Well actually AFAIK it is impossible).