Shp2pgsql.exe should be in folder
C:\Program Files (x86)\Postgresql\9.0\bin.
It might help to include the folder to the systems PATH variable (although it is not here with me).
I don't know how Qgis reacts if you updated your Postgres or Postgis version after installing Qgis.
Does the DB Manager plugin see your Postgis databases?
The plugin help gives this hint:
Note: not having PostGIS installed on your local machine or being installed in some non-standard location will render this feature unusable because of the missing shp2pgsql tool.
According to this message: http://osgeo-org.1560.n6.nabble.com/Re-Qgis-user-PostGIS-Manager-compatibility-issue-with-PostGIS-2-0-td4957715.html#a4958320 the Postgis manager plugin should be replaced by the DB manager.
Unfortunately, the option to import data is still missing. But you can drag and drop shapefiles from Qgis Browser to DB Manager plugin which will do the same.