Index: AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.java
===================================================================
--- AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.java	(revision 610)
+++ AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.java	(revision 611)
@@ -52,4 +52,5 @@
 import net.oni2.aeinstaller.backend.oni.InstallProgressListener;
 import net.oni2.aeinstaller.backend.oni.Installer;
+import net.oni2.aeinstaller.backend.oni.OniSplit;
 import net.oni2.aeinstaller.gui.about.AboutDialog;
 import net.oni2.aeinstaller.gui.downloadwindow.Downloader;
@@ -349,4 +350,32 @@
 	}
 
+	@SuppressWarnings("unused")
+	private void checkDotNet() {
+		if (!OniSplit.isDotNETInstalled()) {
+			HTMLLinkLabel hll = new HTMLLinkLabel();
+			String dlUrl = "";
+			switch (Settings.getPlatform()) {
+				case WIN:
+					switch (Settings.getArchitecture()) {
+						case X86:
+							dlUrl = "http://download.microsoft.com/download/c/6/e/c6e88215-0178-4c6c-b5f3-158ff77b1f38/NetFx20SP2_x86.exe";
+							break;
+						case AMD64:
+							dlUrl = "http://download.microsoft.com/download/c/6/e/c6e88215-0178-4c6c-b5f3-158ff77b1f38/NetFx20SP2_x64.exe";
+							break;
+					}
+					break;
+				default:
+					dlUrl = "http://www.go-mono.com/mono-downloads/download.html";
+			}
+			hll.setText(bundle.getString("dotNetMissing.text").replaceAll("%1",
+					String.format("<a href=\"%s\">%s</a>", dlUrl, dlUrl)));
+			JOptionPane.showMessageDialog(this, hll,
+					bundle.getString("dotNetMissing.title"),
+					JOptionPane.ERROR_MESSAGE);
+			exit();
+		}
+	}
+
 	@DoInBackground(progressMessage = "mandatoryFiles.title", cancelable = false, indeterminateProgress = false)
 	private void checkMandatoryFiles(final BackgroundEvent evt) {
Index: AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.properties
===================================================================
--- AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.properties	(revision 610)
+++ AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.properties	(revision 611)
@@ -58,2 +58,6 @@
 mandatoryFiles.title=Checking for mandatory files
 mandatoryToolsInstall.title=Installing mandatory tools
+
+dotNetMissing.title=.NET is not installed
+dotNetMissing.text=.NET, which is required to use this tool, is not installed on this machine.<br>Please download and install it:<br>%1
+
Index: AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.yml
===================================================================
--- AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.yml	(revision 610)
+++ AE/installer2/src/net/oni2/aeinstaller/gui/MainWin.yml	(revision 611)
@@ -6,5 +6,5 @@
   locationRelativeTo: null
   defaultCloseOperation: doNothingOnClose
-  onWindowOpened: [execDepotUpdate,checkMandatoryFiles,initialize,checkUpdates,focus]
+  onWindowOpened: [checkDotNet,execDepotUpdate,checkMandatoryFiles,initialize,checkUpdates,focus]
   onWindowClosing: [saveLocalData,exit]
   iconImage: img.ae