Bubba is right, try that first. If still a problem, check that your hostname does not contain special characters (this can cause problems). This includes spaces, asterisks (*), underscores (_), ampersands (&), etc.
If that's not the problem, start up Task Manager. these processes should be running:
java.exe
javaw.exe
ArcGISServer.exe
rmid.exe
cmd.exe
conhost.exe
If still a problem, try this:
Stop ArcGIS Server Service
Stop ANY arcgis server services that are owned by the "arcgis user" (check Task Manager)
Start ArcGIS Server Service
Try to create site again.