In the heat of the moment it is natural to want to just Alt-Tab between applications. You may be working in Word and feel the overwhelming need to check your mail. Alt-Tab brings Outlook to the fore, so you Alt-Tab again to get back to your prose.
Doing this can cause Outlook to slow down and even stop. Outlook is still active, just waiting in the background. The suggestion is to minimize Outlook between peeks.
"Get in the habit of minimizing Outlook 2003 rather than just using Alt-Tab or a Windows task bar button to switch to another program. If you leave the full Outlook 2003 window active, it may use so much memory that it stops responding"
"While Outlook 2003 is running, it periodically pages memory in and out of the process address space. When the memory that is free is large, Outlook 2003 is permitted to have more pages in memory. This increases the memory that is being used by Outlook 2003. If another application requires memory, Outlook will release memory.
When you minimize the Outlook 2003 program window, the pages are switched so that the pages that are not required for user interaction are switched out of memory. When you maximize the Outlook 2003 program window again, the pages that are used for user interaction are switched in to memory, and the memory usage begins to increase again.
This could happen when working with a large .pst or .ost file as well as other actions within Outlook."