A specific case of using assembly in C...

In a windows environment, how different is it to set the video mode to 13h for example? This is in comparison to a DOS setting where:

MOV ax, 0013h
INT 10h

...would do the trick.

My aim has been to write decoders of various file formats in QB, C, and Ruby (along with others like C++/C# when appropriate). I have gotten to the point where I would rather not implore a library in C or Ruby when all that is needed is to just set a graphics mode and plot a single pixel when dealing with image file formats. I want to use assembly to perform functions that choice languages do not natively support.

I understand things are different in Windows and wonder how many 'rings' of protection, system calls, (and fire) I would have to jump through in order to achieve something like this with assembly. To keep things relevant to this sub-forum, can anyone give me some directions for doing this in C on an x86 machine?

Looking forward to thoughts on this.