When engineers at Siebel Systems designed the High-Interactivity framework for the brand new High Interactivity Web Client coming with Siebel 7.0 at the beginning of this millenium, they chose a technology which was widely adopted in those days to provide a rich user experience: Microsoft's ActiveX.
Naturally, the only browser which supported it was Microsoft's very own Internet Explorer. And in 2001, when Siebel 7.0 was released, IE had a market share of something between 99.7 and 99.9% (I'm just guessing and maybe wildly exaggerating).
Things have changed, the Siebel forums are full with questions like "does Siebel 7.5 run in Chrome?".
The bad news is that the Siebel High-Interactivity framework depends on IE, period.
The good news is that the good people at Oracle are working like beavers to get Siebel patches out which support the various versions of IE on the various MS operating systems.
Below is a small compilation of information which you might find useful.
The following document on My Oracle Support is a good start to avoid general IE configuration errors.
Siebel version 7 & 8 settings for Internet Explorer versions 6, 7 and 8 [Doc ID 1066053.1]
The next document provides the minimum maintenance release for Siebel CRM when you are intending to use IE 7 on Windows XP or Vista:
Siebel Support for Internet Explorer 7 (IE7) [Doc ID 475275.1]
These are the minimum maintenance release levels for Siebel 8.1,8.0,7.8 and 7.7 to run Siebel CRM HI clients on Windows XP and Windows Vista using Internet Explorer 7 (IE7):
The following MOS document describes the versions for IE 8 on Windows XP (SP3), Vista (SP1) or Windows 7:
Siebel Support for Internet Explorer 8 (IE8) [Doc ID 796015.1]
In short, we must have the following minimum maintenance release levels of Siebel 8.1, 8.0, 7.8 or 7.7 to operate Siebel CRM HI clients in Internet Explorer 8 (IE 8):
Have a nice day