I wish I'd seen this post before you went through the trouble of reinstalls or editing your registry. The settings you are looking for are located in the Selection Menu on the main control panel.
At the bottom are 3 settings that have a profound effect on XSI's selection methods:
-Single Select Object in Region: You must always use the SHIFT key in order to select more than one scene element with any kind of marquee or lasso selection.
-Extended component selection: For components such as points and polys, Left key to select, Middle to deselect, right to toggle. Just like SI3D. In this mode, you can still hold down the ALT button while you right-click in order to access options menus.
-Softimage | 3D Selection Model: A combination of both of the above.
XSI 1.5 will sometimes install and set these on or off based on previous settings, regardless of what selection mode you chose during install. If your selection mode is behaving differently from how you like, go to the select menu and modify these settings.
Hope this helps.