To install on Windows XP:
-Right click on REalPhone.exe and select Properties
-Select the Compatibility tab
-Click Run this program in compatibility mode for
-select Windows 95 in the dropdown.

Now install.

After the install, a window will be present that shows various shortcuts, 
including one to the installed RealPhone program. Repeat the above
operation with that file. That is,
-Right click on REalPhone shortcut and select Properties
-Select the Compatibility tab
-Click Run this program in compatibility mode for
-select Windows 95 in the dropdown.

If you want to select the actual program i.e., instead of the shortcut,
I suspect you will have to do the same thing.