Outlook PST files
Like the saying goes, don't put Pst eggs in one basket. But that's exactly what Microsoft allows you to do starting with Outlook 2007. PST files hold all your email, calendar, and address book (contacts) information. If that file gets corrupted or deleted, you could loose all your emails. Pst files get corrupted all the time.
Actually, Microsoft has allowed users to have large pst files even prior to 2007, but the previous limit was 2GB. Once users hit that limit, all sorts of problems cropped up for Outlook users. Among those problems was the notorious resending of the same email. If let on, it would send out that same email every time it checked the internet. Their friends and coworkers get blasted with hundreds of copies of the same email from the Outlook user.
Where things really got to be a problem was when the pst reached its maximum size and it was corrupt. This happened more often than you'd think.
If you have Outlook, use the SCANPST application that came with Outlook to scan your pst file. I bet you'll find you have a corrupt pst file. Microsoft recognized this file corruption as a problem and for Outlook 2007 they have it check the pst file every time you load Outlook. If you have a small pst file, then you'll likely not even notice the slowdown. However, for those of you that have large pst files, you could experience long delays once you open Outlook.
Long delays in Outlook cause many people to use the task manager to manually crash Outlook. Since Outlook is usually doing a check or repair of the pst file, the chances that the pst file will get even more corrupted increase dramatically when users crash their Outlook. Unplugging or loosing power to your computer during a repair can have the same effect or worse.
If you are using Outlook, try to keep your pst to a manageable size. Periodically check the size of your pst file(s). If any are more than 500mb, create a new pst file and archive those old emails.