Free Registry Cleaner
It seems to me that the performance of Windows-based machines decreases over time. Kind of like a car's engine except you'd think an operating, without any moving parts, wouldn't degrade. This slow-down, as far as I can tell, is due to junk that makes its way on to your machine as part of the many software installs/uninstalls/upgrades/patches that are part of what it means to run Windows. This junk seems to build up to the point where it takes more and more time for applications to load.
One of the prime locations for this junk is the Windows Registry. This is a file that stores thousands of bits of information and links to various files in Windows. Just about every piece of software you use interacts with the registry. When the registry information, and particularly the file references, become invalid, Windows performance decreases.
Fortunately, there are a myriad of registry cleaners out there that help automate much of the registry cleaning process. A new favorite (thanks Scott Hanselman) is CCleaner a.k.a. Crap Cleaner. Running this tool noticeable decreased the load time of Outlook for me. What in particular did it remove to have this affect? I have no idea. It identified hundreds of issues so I threw caution to the wind and let CCleaner take care of everything. Please take note that modifying the registry manually or with a program like CCleaner can result in your machine no longer being usable. I'll leave it up to you to decide if using this registry cleaner is worth the risk.