How to open window xp installed at my computer while standing in my second os window 7?

you can not do that, but you can install a virutal machine on windows7 running an image of windows xp 

switching OS to os (Dualboot) have to restart and select the OS while boot-up, Or Have to install Vertualazing tool

when we start window then find the option for window choice.Select & enter on preferd window

using virtual box we can open windows xp while standing in windows 7

basm allah alrahman alrahim

switch done between accounts

to switch between different OS you must restart your computer or install Different OSs on Virtual box (virtual machine like Safwan Oyoun said )