I just installed a brand new copy of QGIS 1.8 Lisboa and all of the associated software on a Windows Vista PC using the Windows Osgeo4W QGIS installer, and I get this message:
"The Plugin Installer has detected an obsolete plugin which masks a newer version shipped with this QGIS version. This is likely due to files associated with a previous installation of QGIS. Please use the Plugin Installer to remove that older plugin in order to unmask the newer version shipped with this copy of QGIS."
I did have an older copy of QGIS but I uninstalled it and I am 99% certain that I successfully deleted all of the associated files and directories. I notice that the Osgeo4W directories created by the OSGEO4W installer include several versions of QGIS (including 1.0 and 1.4), but I am a novice user and don't know how to figure out which plugin is causing trouble, or if this could be a bug in the Osgeo4w installer.