If I download Ubuntu, and my computer has Windows, how would I start up my OS?

You have to change in BIOS and am start you will have to select between Windows or Ubuntu.

The default install of U. gives a 'boot manager' which looks over your hard drive for all OS's on the drive.

If you install U., when you cold boot, a plain text only screen comes up.

At the top is U., and down from that (@ bottom) is whatever Windows you have.

Using your keyboard arrows, move the highlighted bar to what ever OS you want, then hit 'Enter' key.

U. auto-boots after a few seconds if you do not highlight to Windows.

Using U. as a 'live' CD, your computer boots to Windows as it normally would; you then insert the U. CD, and that 'runs' just like any other application..,it does not 'install' to the hard drive and must be re-done every time you want to use U.

See: http://www.psychocats.net/ubuntu/installing for screenshots on exactly what is involved.

It installs a boot manage so you can choose what to boot up at start.

You should make some free space on the drive first (30-40GB should be fine to start). I usualy use the Easeus utility to do so.

Just pay attention when running the installer. If there is free (unpartitioned) space on the disk the default is to install into that, otherwise the default is to overwrite the entire disk. To be safe use the manual specification. Allocate 2GB for swap space and formate the rest of the free space to ext4 and mount it at /

If you download Ubuntu you should download an iso. You can write it to a DVD or flash stick (or some images a CD) or you can run it in a virtual machine.

You don't have to install it in other words. Or you can as a dual boot -- which you can and should look up.

U can run both OS simultaneously. All the things you need to do is:

1] First install windows

2] Install Ubuntu

Now when you will run your PC/Laptop you will get both the OS, and u can select one of them

you can just put ubuntu on a usb or a disc and boot from it, or install it on a partition and boot from that

I would suggest installing it on a USB drive