Be Careful What You Wish For
It's a good thing that there is the open-source Firefox in existence.
The result of this sort of thing could be that Microsoft not only couldn't include IE with Windows, but that it couldn't give it away for free either. (How would you download it off their web site without a web browser anyways? Install an FTP client from a magazine coverdisk or shareware collection CD, maybe?)
So all the other web browsers would cost money too, like back in the days when Netscape was something that was sold.
It's true that there is a big problem with choice in the computing field. You can't go out and buy modern computers with modern performance that have chips that run the 680x0 instruction set to run your old Macintosh/Atari ST/Amiga software at today's native speed... nor, for that matter, is there a compatible upgrade path that continued without a break to the present day for Apple //GS owners based on a chip compatible with the 65816 but with instructions added to its 16-bit mode for 64-bit addressing, hardware floating-point, MMX- or AltiVec- like vector instructions, and so on and so forth.
But it is rather late in the day to try and bring these choices back now. The x86 Mac, and Linux on the x86, are today's only alternatives, and they're both far behind Windows in the availability of software and peripherals. Yes, by all means prevent Microsoft from obstructing the emergence of choice, but also think about where choices can come from.
If there was an international standard for hardware drivers that had to be followed by commercial operating systems vendors, then anyone wanting to compete would just have to support that standard for all the hardware out there to work on his operating system. Write once, run anywhere - but *not* at speeds slower than native hardware execution speed - is the goal that should be investigated; perhaps some universal subset API particularly for installers and hardware support utilities (so that on a Windows machine, a Mac, or Linux, you could use exactly the same binary to change your screen resolution, which would look like it was written for Windows 3.1 or OS/2 or something... although nothing would prevent hardware manufactures from including prettier ones for the more popular operating systems, admittedly).
While they're at it, they could make a law requiring the x86 architecture to add a feature, like the Itanium has, to switch into big-endian mode.