Couldn't access Quickbooks Online on a XP machine, IE8 fully patched. Despite support advising to 'enable the ActiveX control', I was unable to see/enable the Terminal Services ActiveX control in IE7/IE8 on XPSP3. The fixwas to delete the following registry keys:
HKCU\Software\Microsoft\Windows\CurrentVersion\Ext\Settings\{7390f3d8-0439-4c05-91e3-cf5cb290c3d0}
HKCU\Software\Microsoft\Windows\CurrentVersion\Ext\Settings\{4eb89ff4-7f78-4a0f-8b8d-2bf02e94e4b2}
Once you delete these keys, the ActiveX control should be enabled.