Like all dual-booting processes, you must have ample space in your hard disk to do so, then you have to partition your disk accordingly for each operating system you wish to install.
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Have a look at WUBI, which is a program that allow Ubuntu to be installed as a file on a Windows computer, without making any changes to the Windows hard drive. It can be uninstalled like any other program using Windows add and remove utility.
Yes.
Yes.
Yes.
So that you can choose to load either linux or (usually) windows.
Get Linux!
Dual-boot or multiboot.
many linux distributions you can just download and burn to cd and then boot from cd afterwards and most of them have the dual boot option so you don't need to create one but if you'd like to make one you can use a bootloader like grub or syslinux but most linux distros already have a dual boot option.
Of course you can. The de facto standard bootloader, GRUB, is the reference implementation of the multi-boot specification.
boot the system you need the file to be in, and navigate to the other systems folders.
"Mode?" They're two separate operating systems. If you're in a dual-boot, you simply reboot and select Windows instead of Linux from the boot menu.
Yes you can. Ubuntu's WUBI uses the Windows bootloader (NTLDR) to boot Linux. It also uses the Windows filesystem (NTFS). This, however, is not the traditional way to do things. The traditional way to dual-boot Linux and Windows is to use a generic bootloader such as Grub to boot both Windows and Linux.
Yes. Doing so isn't particularly difficult, and most Linux installation discs can guide you through the process.