Hi indy,
Some game saves consist of just a jpeg with no separate data file - Dracula Origin is such a game, so you found those ones already. I'm unsure as to Murder in the Abbey.
For XP, the first places I look for saved game files are in Program Files or My Documents. Other games are tricky and go the C:\Documents and Settings\Owner\Application Data route.
Those are three most common areas for finding save folders. I'm not familiar with Vista, but it seems there are a whole myriad of problems with the location of save folders with that system.
Most games do retain your saves on your hard drive even after uninstalling the game itself. Otherwise they may prompt you on uninstalling as to whether you wish to keep your saves or not.
If you have trouble finding your saves for any game, feel free to post in Hints and we'll be happy to help.