Solution: This issue may be caused by the Happy99 Virus*. In short, the Happy99 Virus infects a user's system by pretending to be a harmless program that displays on screen fireworks, and by wishing the user a happy new year 1999 (see Figure 1). But, in the background, the program is altering important system files used by windows. After Happy99 has been run, the next time the user tries to send an email message, Happy99 tags along for the ride (this happens without the knowledge of the person sending the email). However, when trying to forward email using certain email programs, most notably Microsoft Outlook Express, the user may run into the above issue with the email program crashing. This is a telltale sign that the system may be infected with Happy99. In order to remove Happy99 from your system, please click here. Note: Happy99 only affects Windows 95, 98, and Windows NT computers. Macintosh, WebTV, and Windows 3.1 and 3.11 are all immune.
 

Figure 1. The Happy99 Sreen.

*Technically, Happy99 is not a virus. Viruses typically infect other programs in order to propagate. Happy99 is actually classified as a worm, which does not have this limitation. Since it also pretends to be a harmless program, it is also classified as a Trojan Horse.