The problem is that I could install rockbox, but not the bootloader (it shows that computer (windows OS) can't find ipod)! Anyway, now when starting it shows error - "can't load rockbox".
Then you have installed the bootloader and the build is missing. If Rockbox Utility didn't recognize the Ipod, how did you install it then?
I used rocbox utility to uninstall rocbox (for bootloader it still shows "can't find"), but it still shows error when starting.
well, that error is shown by the bootloader. Either install a build or remove the bootloader.
Question: how can I totally remove rocbox from ipod so it would boot default ipod OS in this situation?
You could try ipodpatcher, but as Rockbox Utility internally uses ipodpatcher this shouldn't make much of a difference. But it might be worth a try.