2003 slow to preview/view

I've been having trouble with Outlook 2003 for some time now, and I can't
seem to find the fix that applies to my situation.

I use the preview pane, and it is terribly slow to load the previews on html
mail. Sometimes it wipes the whole screen out, showing just a white box with
the blue title bar at the top (which says Outlook Not Responding). If I wait
30 sec. or so, it will finally show the email.

I've added all to my Safe Sender list, thinking that would solve it. No
luck. But, it seems to get better the longer the program is in use, like it's
"learning" the emails or something. If I close Outlook 2003 and start it
again, it's back to slow mode.

I use AVG Anti-Virus and Comodo Firewall Pro, but I've tried disabling these
and that doesn't seem to help.

Any ideas?

System info: AMD Athlon 64X2 Dual Core 4800+, 2.51GHz, 1.93 GB RAM, WinXP
Pro w/Office Small Biz Edition with SP3

Thanks!