This is tutorial is tailored specifically to people who will have to permanently let go of their PC before they can import the data into their Mac, thus relying on an external hard drive. This happened to my father in law (suegro) who was exchanging his PC for a Mac in a store, and could not wait for them to do the data transfer themselves.
For cases like these, I add some steps as fail-safes, but as the data in your PC will never be available to you again, better be safe than sorry. It's not hard, and all you will need is an external hard drive (HDD) and diligence.
1. YOUR SOFTWARE
In your PC, go to Control Panel / Programs / Uninstall. This will give you a list of all of the software installed. Skip the ones that come included with Windows and write down which ones you plan to use, as you will that way know which to install when you get your Mac. In this case, we retained Itunes, MS Office (including Outlook), Skype, Picasa, Google Chrome, Blackberry Desktop, and Kaspersky Antivirus.
2. YOUR DOCUMENTS
Documents are any files that are not emails or software. These should be your music, pictures, Word and Excel, etc. For Windows 7 users, documents will usually be limited to your Desktop and Librairies. In Librairies, they will usually be in Documents, Music, Pictures, and Video.
Copy all of those files to your HDD and keep them organized by folder. This makes it easier to check whether the folder sizes on your HDD and computer match.
3. YOUR OUTLOOK EMAILS
No matter your email protocol (POP, IMAP, or Exchange), I recommend doing all of these steps. In theory, it should only be required for POP emails, but who wants to take those kinds of risks when the data will be lost to you forever after you part with your PC? Now is also the time to disable automatic send/receive settings you have, so that you do not download emails during the process. Likewise, once you begin the process, you should stop using Outlook to receive or send emails.
3.1 The Outlook Data Files
Go to your account settings in Outlook, and in the Data Files tab, you will see the path to all of the files that are being used by Outlook. Locate those files, and copy them to your external hard drive. This is a fail-safe, as you will probably never use those files again.
3.2 Your Account Settings
For your POP or IMAP addresses, go to your Account settings in Outlook and list all of the data you can about your email addresses, including the advanced options. You could also screen shot every tab, and copy/paste to a document that you'll save on your HDD.
For your Exchange server addresses, you need to first close Outlook, and then go to Control Panel / Mail. Due to the high amount of tabs and data, the print screen method will probably work better for you here.
3.3 The Outlook Data Conversion
You need Outlook running to do the data conversion. There is a free method for this and there are paying software available.
The free method uses Mozilla's Thunderbird. All you need to do is import your emails into Thunderbird, and then export them into .mbox files. Many users have been very successful with this method. My suegro's Outlook had a glitch that prevented me from using this method, but it should work fine for you.
Little Machine's O2M ($13 at time of writing this post) had worked great for me in the past, but it couldn't get around the Outlook glitch.
I then used MessageSave 5.0 ($50 at time of writing this post) successfully. It exported all of the folders and subfolders in the Outlook files. Make sure you run the software as many times as you have Outlook files. The export process is seamless with MessageSave (I ran it for Calendars and contacts as well).
Make sure the converted data is on your HDD.
4. DOUBLE CHECK YOUR DATA
The quickest way to check your data is tally up the folder sizes on your PC and see how it matches with what you have on your external hard drive. Do not look at total disk usage space, as your computer is using memory for the operating system and the software. Really make the effort of going through your folders. This may be tedious, but your data is probably more valuable to you than the 5 minutes you'll spend doing this. This is also why keeping the files organized by folder is a good idea.
Also, open some files from your hard drive at random to make sure that they are working. If the copy was interrupted at some point, your data may have become corrupted. Again, this is a 2 minute process, but will save your data if there was an issue. This is also true for your email conversions. The total conversion should weigh roughly as much as your Outlook.pst files. If it's significantly different, then you probably had a fault conversion.
5. BREATHE IN AND OUT
You can finally relax. If you double checked all of your steps, you should be safe. You may feel a little nervous once you let your PC go, and that's normal. Should and Will mean very different things. Also bear in mind that your HDD is now invaluable until you load the data onto your Mac. My recommendation is to leave it alone, to not move it around too much. Treat it like crystal. Hard drives are sensitive, and they can break if jolted.
Also, this method worked perfectly for me, and its principles could be widely applied, but each case is different. As always, do your homework, and happy conversion!