Sean,
If you're not using it you should probably remove it from your system
since it will continue to sync in the background. I'm not sure why the
installer would produce an error, but you can remove it manually by
deleting these folders:
/Library/PreferencePanes/SpanningSync.prefPane
/Library/Application Support/SpanningSync
~/Library/Application Support/SpanningSync
"~" refers to your home folder.
After doing so, logout and log back in to your Mac OS X account.
Please let us know if that doesn't address the problem or you have
other questions.