It might be an overloaded outlook. I use Outlook Express XP and I've no problems.
I've experienced this problem in the past, however. When I used Windows Millenium. I simply started using another e-mail client (like Outlook), you can try Thunderbird (
http://www.mozilla.org/products/thunderbird/) and there are others like Opera's Mail client, I have opera but I don't use that.
My recommendation is to check your computer, no other clients have reported this issue to us hence it doesn't seem to be a 'general problem'.
See if thunderbird fixes it, check your system for viruses and malware. Maybe other members can recommend you another good program for mail.