Windows XP can drive you nuts with the idiotic Passport reminder balloon. If you don't use Microsoft Messenger for your IM needs, or use a different client (see Replace Windows/MSN Messenger for some examples) then you might want to disable that reminder.
Start Registry Editor (Regedit.exe).
Note: As always, before making changes to your registry you should always make sure you have a valid backup. In cases where you're supposed to delete or modify keys or values from the registry it is possible to first export that key or value(s) to a .REG file before performing the changes.
Locate the following key in the registry:
On the Edit menu, click Add Value, and then add the following registry value:
Value Name: PassportBalloon
Value Type: REG_BINARY
Data: 0a (hexadecimal)
Note: 0a as in "zero a" not "oh a"...
Close the registry editor.