When I install Ubuntu, do I keep access to all my Windows Vista files (Word, Excel, etc.) and documents (photos, etc.), and do I keep all my Outlook data (history, address list, etc)?
It depends on how you install Ubuntu. If you format your hard drive and then install Ubuntu, nothing will remain of your Windows installation.
However, if you install Ubuntu alongside Windows, then you will be able to access all of your files (such as photos and videos) from Ubuntu.
This doesn't necessarily mean that you'll be able to open all of your documents, however! LibreOffice (or OpenOffice, depending on which one you prefer) can open most (if not all) files made with Microsoft Office. One should still always test out opening their documents in LibreOffice before completely deleting Windows and Office, just to be safe.
As to Outlook, the answer is that, though it is not done automatically, it is possible to import your settings and mail from Outlook, into Ubuntu's mail client, named Thunderbird. For instructions on how to do so, read the article mentionned in the following paragraph.
I highly recommend that you read this WikiHow article for detailed steps that will help to get you up and running with Ubuntu. It explains in a very detailed fashion how to make the jump from Windows to Ubuntu. It deals with Office and Outlook alternatives and how to set them up, and also mentions a ton of other software you may end up finding useful.
If you choose to install Ubuntu alongside Windows, then you can access your files on the Windows partition from Ubuntu, but you will not be able to read the files stored on the Linux partitions from Windows (at least not without installing drivers). However, you may set up an NTFS partition for sharing files between the operating systems, as this avoids messing up the Windows system files and preventing it from booting (never happened to me, but I read it was better to do this).
Creating a partition for sharing data between Ubuntu and Windows
What I did, in first place, was to back-up my personal files (My Documents, Pictures, etc) on an external hard drive.
Then, I used the Ubuntu installation wizard to install it alongside Windows, and let it do the partitioning. Then, from Windows, I used EaseUS Partition Master to create an additional NTFS partition for storing data (I labelled it Data). I shrank the Windows one to make space for it. Then, I logged into Windows, went to my personal folder, and right-clicked My Documents. On the dialog, in the Location tab, I chose a folder on the newly created partition (I chose E:\Users\MyUserName\Documents, where E is the drive letter for the Data partition). Then, Windows moved all the files to the new location. I did this with the other folders, such as Pictures, Videos, Music, Downloads, etc. They could be then accessed from the start menu or the personal folder as if nothing had changed.
Reboot, and from Ubuntu, follow this tutorial (link) to automatically mount your data partition on start-up. Reboot. A hard-drive icon should appear on the Launcher, indicating that the partition is mounted (mine shows a tooltip saying "Data" when hovering it).
From Ubuntu, I opened my home folder with Nautilus (the default file manager) and deleted the Documents, Pictures, Videos, Music and Downloads folders (they were empty). Then, from the terminal, I created symbolic links to the folders on the data partition:
And similarly for the other folders. Now they appear as if they were in the home folder, but they are actually stored on another partition.
Importing Outlook contacts and mails
Ubuntu 12.04 comes with Thunderbird installed, so if you want to keep your contacts and mails, you will have to export them from Windows and then import them with the Thunderbird wizard in Ubuntu (Tools > Import...).
Boot Windows, open Outlook and export the contacts and mails, and save the files (.csv for the contacts) somewhere in the Data partition (for example in E:\Documents\Outlook_export\ , where E is the drive letter for the Data partition). Then reboot and select Ubuntu, and import those files from Thunderbird, from ~/Documents/Outlook_export/ .
Hope this helps :)