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...