On my XP system, Five different users each have their own account. The log-on window shows the number of e-mails that are unread, but it's totally inaccurate. One time it reads 30, the next 2, the next 10, and so on. But actually all e-mails have been read and are marked as such. How do I turn this useless thing off?

TIP:
Various e-mail clients put information on unread messages in the Windows XP Registry, but this information isn't always correct. To clean up the situation, you'll first clear the existing information and then configure the system so it doesn't add more.

Launch RegEdit from the Start menu's Run dialog and navigate to the key

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\UnreadMail.

You'll find one or more subkeys below this key.
Select each subkey in turn and find the value MessageCount in the right-hand pane. Double-click that value and set its data to 0.
Now look for a value named MessageExpiryDays.
If you don't find it (and you probably won't), choose Edit | New | DWORD Value from the menu, name the new value MessageExpiryDays. Set this value's data to 0.
If there are multiple user accounts on this computer, you'll need to log on as each user and repeat this process.

Just to cover all bases, navigate to

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion

and look for a subkey named UnreadMail.
If you don't see it, go to Edit | New | Key to create it.
Open the Unread-Mail key and find or create the MessageExpiry-Days value as before. Set its data to 0.
This needs to be done only once, not once per user.