Added workaround to allow installation on Windows XP x64-Edition again: It appears that NSIS (intentionally!) detects Windows XP x64-Edition as "5.1" although it actually is "5.2", so the Windows XP (non-x64) Service Pack checks were triggered on Windows XP x64-Edition too. And, as there is no such thing as a Service Pack 3 for Windows XP x64-Edition, installation was aborted. Consequently we will now avoid the Service Pack checks, if running on a x64 system.

etc/NSIS/setup.nsi

@@ -75,6 +75,7 @@
 
 !include `MUI2.nsh`
 !include `WinVer.nsh`
+!include `x64.nsh`
 !include `StdUtils.nsh`
 
 
@@ -269,26 +270,25 @@ Function .onInit
 		Quit
 	${EndIf}
 
-	${StdUtils.GetParameter} $R0 "Update" "?"
-	${If} "$R0" == "?"
-		!insertmacro MUI_LANGDLL_DISPLAY
-	${EndIf}
-
 	; --------
 	
+	# Running on Windows NT family?
 	${IfNot} ${IsNT}
 		MessageBox MB_TOPMOST|MB_ICONSTOP "Sorry, this application does *not* support Windows 9x or Windows ME!"
 		Quit
 	${EndIf}
 
+	# Running on Windows XP or later?
 	${If} ${AtMostWin2000}
 		MessageBox MB_TOPMOST|MB_ICONSTOP "Sorry, but your operating system is *not* supported anymore.$\nInstallation will be aborted!$\n$\nThe minimum required platform is Windows XP (Service Pack 3)."
 		ExecShell "open" "http://windows.microsoft.com/"
 		Quit
 	${EndIf}
 
+	# If on Windows XP, is Service Pack 3 installed?
 	${If} ${IsWinXP}
 	${AndIf} ${AtMostServicePack} 2
+	${AndIfNot} ${RunningX64}
 		MessageBox MB_TOPMOST|MB_ICONEXCLAMATION "This application requires Windows XP with Service Pack 3 installed.$\nWindows XP *without* Service Pack 3 reached end-of-life on 2010-07-13.$\nCurrent Windows XP (Service Pack 3) will be supported until 2014-04-08.$\n$\nPlease install Service Pack 3 now or just run Windows Update!"
 		MessageBox MB_TOPMOST|MB_ICONQUESTION|MB_YESNO "Do you want to download Service Pack 3 for Windows XP now?" IDNO +2
 		ExecShell "open" "http://www.microsoft.com/en-us/download/details.aspx?id=24"
@@ -297,6 +297,13 @@ Function .onInit
 	
 	; --------
 
+	${StdUtils.GetParameter} $R0 "Update" "?"
+	${If} "$R0" == "?"
+		!insertmacro MUI_LANGDLL_DISPLAY
+	${EndIf}
+
+	; --------
+
 	UserInfo::GetAccountType
 	Pop $0
 	${If} $0 != "Admin"