Is there any document with description which registers does each API use?
I'm interested in APIs in WinXP.
Or the one way to find it - is decompilation of each function ?

All Windows APIs can/might/will use EAX, ECX and EDX at anytime. All other registers are preserved.

Thank you. But that I already know.
I'd like to know if some API, for example, does not use EDX, how can I find it without decompillation of every API? Is anywhere such information?

I've never seen any official documentation and I doubt I ever will. The OS is written in HLL and the compiler generally doesn't tell you what functions are not using particular registers.

But, be that as it may, what you propose is risky. Your code can break at any time with a new security patch or service patch or upgrade or whatever. And what you might gain, if anything, is of dubious value.

Thank you very much. Now I've got the idea
You are a real brother-in-asm =).