All mail in Outlook 2000, XP, and 2003, are stored in a single file labeled outlook.pst. Mike is correct that with 2000 and XP, the PST file size can be no larger than 2 GB. If it breaches that size, it will refuse to download any more mail.
However, if you have trouble with Outlook opening, there is a chance that the program crashed and a TEMP file became corrupted. The other issue is that I believe sometimes Outlook needs to re-index the PST file so it knows what mail is where. The larger the size of the file, the longer it will take to open Outlook after a crash. I would associate a long start time, however, with a potentially corrupt temp file.
Hope that helps!
Ariston Collander
ariston@coxcomputer.com