How do I get the install directory for the version of the runtime that is loaded in the current process?

For example, the ‘C:\WINNT\Microsoft.NET\Framework\v1.0.3705’ dir for the 1.0 framework version.

You can do so using PInvoke to call this native API method:

[C#]
[DllImport('mscoree.dll')] 
// Declaration
internal static extern void GetCORSystemDirectory([MarshalAs(UnmanagedType.LPTStr)]System.Text.StringBuilder Buffer,
	int BufferLength, ref int Length);
// Gets the path to the Framework directory.
System.Text.StringBuilder sb = new System.Text.StringBuilder(1024);
int size;
// returned value in size can be ignored
GetCORSystemDirectory(sb, sb.Capacity, ref size);

[VB.Net]
’ Declaration
Private Declare Function GetCORSystemDirectory Lib 'mscoree.dll' ( ByVal Buffer As System.Text.StringBuilder, ByVal BufferLength As Integer, ByRef Length As Integer) As Integer

’ Gets the path to the Framework directory.
Dim Path As New System.Text.StringBuilder(1024)
Dim Size As Integer
’ returned value in Size can be ignored
GetCORSystemDirectory(Path, Path.Capacity, Size)