To troubleshoot an installation, sometimes I just want a quick answer to what version of .NET is installed.
Is there a way to determine the .NET Framework version on
Based on your update which indicates this is for walking a non-tech savvy end user through it, I suggest going to Scott Hanselman's site http://www.smallestdotnet.com (use Internet Explorer) which uses the user agent string to tell you which .NET Framework you've got and gives you recommendations for getting up to the latest version in the most efficient manner.
Old Answer
With PowerShell you could do this (although the presence of PowerShell already implies at least .NET 2.0)
Get-ChildItem "$($Env:WinDir)\Microsoft.Net\Framework" -i mscorlib.dll -r |
ForEach-Object { $_.VersionInfo.ProductVersion }
I don't know if there's a comparable way to get the version information in plain old crusty cmd.exe.