This problem depends on many things.
Are you the only person on your LAN having this issue?
Ask your netowk admin first: have you run Office updates on your PC? Go to office.mircosoft.com and look for updates button.
Do other network capable programs have a slowness issue?
Have you tested signing on from a different PC on your LAN with your credentials? (You should try this first to eliminate your PC.)
Has the A\V scanner changed in your organization?
Does this happen at a specific time of day?
Do you have anti-virus running on the Exchange server? When is it set to scan?
Do you have a SMTP virus scanner and general anti-virus scanner on the Exchange server? Did you exclude the SMTP scanner folders from the A\V real-time scan?
Is the Exchange server on your LAN or remote over a WAN connection?
Has your DNS changed recently? Do you use a HOST file and DNS server(s)?
Is there an out dated add-in in Outlook 2003 for your anti-virus?
Have you noticed a speed change in your network?
Are you running 10 mbps or 100 mbps on your network card?
Have you tried to change your port connection the network switch?
Find the answers to some of these things first.
<b>Ask your network admin first:</b>
Open command prompt, type ipconfig /flush dns, hit enter. May not help, but doesn’t hurt.
Do you have a HOST file? Search and open with Notepad. Make sure the entries are correct. Make sure your DNS setup is correct.
Run Outlook repair from Add/Remove programs.
Hope this helps.