Step 1: Consider turning off virtual memory if you can affort it (see previous blog entry): Right click MyComputer, Properties, Advanced, Performance - settings, Advanced, Virtual Memory - change, no paging file, reboot (or wait until step 2 completed before rebooting).
Step 2: Right click MyComputer, Properties, Advanced, Performance - settings, Advanced, set memory usage to system cache, reboot. This the first thing I do on new computers with more than 256MB RAM. It sets up the machine to keep items in memory in a unified cache. Try searching Outlook email twice (different search criteriea) and you will see that there will be almost no disk activity and an order of magnitude performance increase for the second search. I have only had trouble with this setting once due to a malignant ATI notebook driver so you may want to check that your drivers are fresh before committing, but it is worth it.
Step 3: See the deal about defragmentation in the post below