Toad 11.5.1 installer appears as not signed
The Digital Signatures tab may not appear in the properties dialog box of a digitally signed file that is larger than approximately 400 MB in Windows XP with Service Pack 2
The installer is properly signed. Microsoft has confirmed that this is a problem in the Microsoft products.
Please refer to http://support.microsoft.com/kb/922225/en-us for more information about this.