I thought a few weeks ago someone asked a question about running multiple versions of qgis on the same computer. But I can't find it back.
When you install Qgis standalone version on your computer, it installs in a folder with release name under *C:\Program Files*. Therefore it shouldnot be a problem.
When you install the following versions, they go in the respective folders
QGIS 1.8.0 Release --> Quantum GIS Lisboa QGIS 1.7.4 Release --> Quantum GIS Wroclaw QGIS 1.5.0 Release --> Quantum GIS Tethys
I hope it helps you...
You can have multiple versions of QGIS installed through OSGeo4W. One is called qgis, the other qgis-dev (in advanced installation).
I've been running OSGeo4W on both Win XP and 7 with both release and developer version installed and never had any problems.