Same question as Make Safari open new links in existing windows as a tab, rather than a new window (for Mac OS X), how can I do this with Windows?
Example, every time I open a link in Google Reader a new window is created, I would like that window to be created as a tab and not a new window on Windows. Can this be done?

%APPDATA%\Apple Computer\Safari\Preferences\com.apple.Safari.pliston Windows does not actually yield the desired results with Google Reader. External apps do respect the preference in the General section. – Chealion Aug 6 '09 at 20:17