Dual Boot Simultaneously

i am currently dual booting Windows XP and Ubuntu linux, but i hate having to restart my computer every time i want to switch OSes. is there any way to have both windows and linux boot at startup, and “log in” or out of one? like if i logged into windows and then later that day wanted to switch to linux, i want to just click to log out of windows, and go right into linux quickly. it seems like that should be possible, even though the RAM and CPU and everything would be sharing resources. does anyone know how this would be possible, or why it wouldn’t be?