I hate to tell you, but the best way to learn how to do it from scratch is to look at what has already been done by others.
If you truly want to do it "just straight from scratch" then there is no software out there to help you. You have to do it yourself.
------------------------------------
Additional Information:
Nobody has written an OS from scratch in 30 years. Just like you couldn't make a hammer without a tool to forge the steel and trim and cut the handle, you don't make an OS from scratch.
The most basic OS would be written in assembler (machine language) and compiled. Then you're writing in mnemonic op code and compiling it down to an executable. You would still need an assembler to write the program in.
Microsoft.
it is Microsoft windows
Windows XP is an excellent operating system and can be purchased and downloaded online. I would go straight to the source, Microsoft. Here is a direct link to the Windows XP download from this company: http://windows.microsoft.com/en-US/windows/products/windows-xp
The computer software Windows CE is available to download onto your computer from the Microsoft website as Windows is a part of the Microsoft company.
Microsoft and Windows.
The software produced by the company ReadPlease was a text to speech converter software made for the Windows platform. It shared the same name as the company and was discontinued in 2003.
Bill Gates is the founder of the software company known as Microsoft.
Microsoft corp.
C Software is a company who makes applications for windows programs. You can learn more information about them at www.c-software.net.
Microsoft is a large software company responsible for such things as Windows, Office, XBox, MS-DOS, and software monopoly abuse.
Bill Gates invented Microsoft windows.
One can re-install the Windows program on any personal computer. Microsoft's website allows users to download the software straight from their website.