How can I found out for which architecture is the libc6 in my linux compiled? And how can I compile(or change) it to another architecture(for example from i686 to i386)?

uname -m gives the machine architecture
uname -p the processor type(either i386,i486,blah blah)
uname -a gives everything
(anyway beats me ,but why are you asking this question coz any i386 program,library et al will work in i686 and I don't think that you have a i386 do you?)

No,I have got a P4,2.4Ghz.But it is a laptop and I cannot compile kernel 2.4.21 for it(I can but then it freezes at the begining of nooting(I had here few threads here but in none I recieved solution)). In the menuconfig I found one option-Unsynced TSC support(it doesn't solve it),but in help for it is written that it is for laptops and other and some NUMA multi-node boxes. And most importnt why I made this thread:
In the help: If your system hangs when init should run, you are probably using a i686 compiled glibc which reads the TSC without checking for availability. Boot with "notsc" and install a i386 compiled glibc to solve this problem.
So now I am trying to find out how can I run the 2.4.21 kernel on my machine(Toshiba Satelite 2435, Debian distribution)

First do you compiled the glibc or you installed them from debian cd?

I don't know that I have been compiling glibc. I was instaling it from the netinstal cd and then instaling all the packages from net.

If you installed glibc from the Debian CD, it will be the baseline 386-optimized glibc. But, if you are still curious, here's the real command to find out the architecture your glibc was compiled for (the uname stuff is the wrong answer).

I have the same result as you,so it's really an i386 architecture. But now I really don't know what to do with that kernel thing,this was my last idea of what could be wrong with my setup. Don't anybody know what should I do to get the 2.4.21 kernel running on my computer?

Which versions of linux will have libc 6.1 ?