Can’t add "Terminal Services Client ActiveX Control" in IE 8

[guest]

Problem

When trying to access windows SBS 2008 RWW, some clients receive this message: "The Microsoft Terminal Services Client ActiveX Control (also known as Microsoft RDP Client Control) is either not available, or is not enabled". The screenshot can be found here: http://www.chicagotech.net/sbs/tscadd-on1.htm
2. So far we have tested 3 computers (other computers running IE 6 or 7 work fine), one is XP with SP3 and IE 8 and other 2 other windows 7. All computers are running RDC v6.1. The screenshot can be found here: http://www.chicagotech.net/sbs/tscadd-on1.htm

3. All these computers don’t have "Microsoft Terminal Services Client ActiveX Control" in IE 8 Add-ons. I have tried to run regsvr32 mstscax.dll which is successful loaded. The screenshot can be found the same link mentioned above.
Why can't I add the "Terminal Services Client ActiveX Control" in IE 8?

[guest]

Forgot to mention that "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Ext\Settings\" doesn't have {7584C670-2274-4EFB-B00B-D6AABA6D3850}. I also tried article: Display Microsoft Terminal Services ActiveX Control (or Microsoft RDP Client Control) - http://technet.microsoft.com/en-us/library/dd701037(WS.10).aspx and http://go.microsoft.com/fwlink/?LinkId=103719

[guest]

1. Upgrade RPD client version.
2. Register Mstscax.dll by following this link: How to install Remote Desktop ActiveX Control in Windows 8 - Step by step with screenshots - http://www.howtonetworking.com/win8&2012/win8-38.htm
3. Reset IE.
How to reset IE - HowToNetworking.comHow to reset IE. In a case, you have an IE problem such as IE crashing, freezing,
reset IE is one of resolutions. To use Reset Internet Explorer Settings (RIES) in ...
http://www.howtonetworking.com/Internet/resetie.htm
4. Please add the RWW URL in the IE trusted zone.