I have used OSGEO4w as my installer for QGIS. I've noticed that it loads both v1.8 and v1.9. I don't have any issues with that except that I prefer to use the more stable v1.8.
I've noticed that when I open a pre-existing project file (.qgs) it defaults and opens in 1.9 (i.e. I open the project by double clicking on the project file. This is different than opening the QGIS desktop interface first and then loading the project file. The latter method works because I open v1.8 first). I checked my file association (Windows 7) for .qgs and it has 'qgis-dev' which is fine, but I'm wondering how I can get it to open in 1.8 instead of 1.9?