Welcome to the forum, MarkC.
This problem is because :
When you try to restore on different PC/OS, although "Restore" can work success (copy data to correct place), but because the old rainlendar2.ini had recorded the old user data path, which may differ with current (new) user data path, so you must manually modify it.
See the attached image, try to change the values of "Filename" to "default.ics"
or "[User folder]\default.ics" or where the ics-file is at
The [User folder] is (Default):
Windows XP: C:\Documents and Settings\[yourname]\.rainlendar2
Windows Vista & 7: C:\Users\[yourname]\.rainlendar2\