how many Operating System can i have on one computer?

[dlx]

can i boot to more than 4 OS on my computer?
i already multiboot my computer with XP, VISTA, 7 and theres is another partition on the pc which is the HP Recovery Disk. i would like to add ubuntu as another OS on my pc, how can i do that?
how can i boot from an extended partition.

[camerongray]

It should be possible.  Ubuntu is good at detecting the oter OSs and adding them to it's boot menu.  Just make sure to have a backup.   

[juzhuo]

can i boot to more than 4 OS on my computer?
i already multiboot my computer with XP, VISTA, 7 and theres is another partition on the pc which is the HP Recovery Disk. i would like to add ubuntu as another OS on my pc, how can i do that?
how can i boot from an extended partition.

Why not try Virtual Machine?

[soybean]

Why not try Virtual Machine?

Good suggestion.  dlx, if you don't know anything about VM (Virtual Machine) and using it to install Ubuntu or other OS inside Windows, see How To Install and Run Ubuntu Linux Inside Windows.

[2x3i5x]

Although you probably don't want to, you can install 145 operating systems on a single computer