Like most people out here, I'm not really a fan of Microsoft Outlook since I know how big, fat, and sluggish it is. I used to ask ones that use Outlook regularly why you have to use and how you live with it. The answer I always got, unsurprisingly, is they didn't know either whether they had another choices or they need to use it with an Exchange account.
Either ways, that's what I have to deal with also; many of my clients want to get things done in their own ways. In other words, using in the same--familiar--environment. Although they want to speed thing up, learning new things--like switching from Outlook to Thunderbird--is much less productive--than slow Outlook that they used to. Thus, the way to improve overall speed & performance is not that much--yes, one way is doing clean installation, but it just normally scares people when talking about this. Tweaking all configuration & clean all messes should be preferred path in any cases.
In order to fix any possible problem which might occur, the best way is to be with it--understand it--by myself. Therefore I could get the most from it. I will write a series of switching to Outlook based on my usage--normally with Thunderbird. You'll see, in the end, if Outlook is worth switching from Thunderbird in my scenario.
From what I have experienced, most of my clients have >4GB mailbox with Outlook and it ran really slow. More and more stuffs makes things slower; it's just the fact, we couldn't do anything about it. What we might try to alleviate this problem is separating the big file into smaller pieces. It's sadly that I still could find the way to separate an Outlook file in case we are using on multiple machine. If you are using on a single machine, there is a good way to do this =) As you will see later on.
Nonetheless, the first important thing we should have done with Outlook is backing up! It would be great if you know that you would never lose all your important archives--thousands of e-mail in Outlook specifically. This leads to the take one: all about backup and file location of Outlook.
For the traditional way, you can use import/export feature in Outlook which is a straight forward. Anyone could do that. However, this feature is not really built-in (from/to another format, besides .pst)--some might not able to do this without an install disc. So, an alternative will take this role--just find its source and copy it!
Basically, Outlook data has 2 main parts: mailbox & auto-complete e-mail list.
1. Mailbox -- Outlook will separate file by mail folder. For example, it always has Personal Folders--this stores in "outlook.pst" file as a default. If you have an archive, it will be "archive.pst" or if you have IMAP account, it will be each file (.pst) for each account too. All this will store in:-
%localappdata%\Microsoft\Outlook or C:\Users\<username>\AppData\Local\Microsoft\Outlook or
C:\Documents and Settings\<username>\Local Settings\Application Data\Microsoft\Outlook
While the first one is of Vista and the latter is of Windows XP. %localappdata% varible is only valid in Windows Vista though.
2. Auto-complete e-mail list file -- this might be the vital stuff for some people. It is stored in Outlook.NK2 file, but in different place which is
%appdata%\Microsoft\Outlook or
C:\Users\<username>\AppData\Roaming\Microsoft\Outlook or
C:\Documents and Settings\<username>\Application Data\Microsoft\Outlook
%appdata% variable can be used in both Windows XP and Vista in this case; which gives you a different location by each OS--AppData\Roaming\ for Vista and Application Data\ in XP
If you keep these files safe, you will have no worry at all!
In case you find the popup,
The data file 'Personal Folders' was not closed properly. The file is
being checked for problems.
You may take a look at the utility coming with Office 2007, "SCANPST.EXE" in C:\Program Files\Microsoft Office\Office12, you will be able to fix that without opening Outlook. It might not operate faster, but surely better than having frozen Outlook on the background =) I'm not quite sure if this is available separately in Office 2003 or not though. Thus, I wrapped this program (2MB) up for grab here, just in case.
So far, you are not likely to lose your data. Then we will talk about how to use it in the next part!
In brief, Outlook has good solutions enough comparing with Thunderbird. It might not cover every settings like backing up Thunderbird profile's folder, but it's up to par in my opinion. Thus, Outlook 1, Thunderbird 1 in this round.