Mention more versions of Windows. (Not Vista, yet.)

[originally from svn r7127]

windows/README.txt

@@ -15,9 +15,9 @@ Start menu.
 If you want to use the command-line-only file transfer utility PSCP,
 you will probably want to put the PuTTY installation directory on
 your PATH. How you do this depends on your version of Windows. On
-Windows NT and 2000, you can set it using Control Panel > System; on
-Windows 95 you will need to edit AUTOEXEC.BAT. Consult your Windows
-manuals for details.
+Windows NT, 2000, and XP, you can set it using Control Panel > System;
+on Windows 95, 98, and Me, you will need to edit AUTOEXEC.BAT. Consult
+your Windows manuals for details.
 
 What do I do if it doesn't work?
 --------------------------------