I have installed Ubuntu for a lot of people because it has apparently been too 'tricky and complicated' for them. Often people want to keep their existing Windows installations, and as such, I recommend installing the OS on a virtual partition. Firstly, I should tell you that emulating an operating system takes up quite a bit of memory. I'd say you should at least have 512 on your PC, and preferably 1Gb or more.

Now, download Ubuntu here, and VirtualBox here. Ubuntu downloads as an ISO file, do not run it yet! Even if you do not know what an ISO file is or does, it does not matter as VB sorts it out for you.

After you have downloaded it, open up the VirtualBox setup you downloaded, and install it (keeping the default options unless you have any reason not to do so). You may get a notice from Windows Security about installing a USB controller and network adapters, allow them to be installed. Once that is done, open up the installation file for Ubuntu. I can imagine that due to the huge difference in size, you are probably still waiting for it to finish downloading - so do something else until it is finished.

Now open VirtualBox. It will probably prompt you to register, so do so unless you have a reason not to. Then, when that dialog is gone, select 'New'. Click next and then type the name as 'Ubuntu Linux'. Then, select the 'OS Type' as 'Linux 2.6'. Then it asks how much RAM should be allocated to the virtual machine; by default the slider is at 256Mb, which should be fine, but it you have memory to spare do use it as it will improve performance. Then it will ask you for a 'Virtual Hard Disk', so select 'New' and choose 'Dynamically expanding image'. It will ask you then what size you want the HD size to start off at. Ubuntu needs 5Gb, so choose 5Gb if space is rife, or choose more if it isn't. Then click 'Next' and subsequently 'Finish'. Then select 'Next' again, and then 'Finish'. At this point it may seem like the wizard has frozen, but it has not, give it a minute or two to create what you have asked for.

Once this is done, select 'Settings' and then the 'CD/DVD-ROM' tab. Tick 'Mount CD/DVD drive' and then select 'ISO Image File'. Then click 'Add' at the top, browse to the ISO you downloaded, and select 'Open'. Then, when it goes back to the previous window, click 'Select' and then 'OK'. Then proceed to highlight 'Ubuntu Linux' and then click 'Start'. Then follow instructions to install Ubuntu.

Once installed, will the user be greeted with a dual boot screen at start up, or is Ubuntu accessed from within Windows?

Ubuntu is installed into a virtual hard disk which is actually a file on the HD itself. This HD and setup can be read by VB which then operates it like it were a new computer.

So yes, it is accessed from within windows, which can be a wonderful thing for someone wanting to get into Linux, but doesn't want to take the risk of undergoing setting up dual booting.

Virtualbox is also availible from here...
Downloads - VirtualBox

I chose the softonic mirror because the main site is down all the time, just to let you know.

I wouldn't recommend running a virtual OS with any less than 1 GB of RAM, 2GB if you plan to actually do something on your computer. I think I would allocate between one third and half of your RAM to the virtual machine, depending on where the most intensive usage will be: in Windows or in Linux.

Very nice guide by the way

If you want to run them at separate times, often as long as you leave 256mb or so RAM to Windows OS then you're sorted.