SSE Not Supported

A popup was added on March 23, 2010 as part of Patch 3.3.3.
"This system will not be supported in future versions"
If you see this, its either 1 of 2 things:
# Your BIOs isn't set up correctly
# Your system is so outdated, it'll no longer be supported come Patch 4.0.1.
:SSE Not Supported
:Due to the evolving nature of the game, the minimum system requirements for World of Warcraft, the Burning Crusade, and Wrath of the Lich King may change over time. One of the upcoming changes will require that your processor (CPU) have SSE support to run the game. The following processors do not have SSE support:
 
:AMD Athlon (Thunderbird-core)
:AMD Duron (Spitfire-core)
 
:If you have a Thunderbird-based Athlon processor or a Spitfire-based Duron processor, you will have to upgrade your computer in order to play the game. Please keep in mind that due to the age of these processors, finding an upgrade may prove difficult. Some original Athlon processors are Slot A processors and do not have an upgrade path to an Athlon XP processor. Socket A/462 may or may not be able to take Athlon XP processors, depending on the motherboard.
 
:Virtually all other processors support SSE. Those include, but are not limited to, the following:
 
:AMD Athlon XP (all)
:AMD Duron (Applebred-core)
:AMD Athlon 64 (all)
:AMD Athlon II (all)
:AMD Sempron (all)
:AMD Phenom (all)
:AMD Phenom II (all)
:Intel Celeron (Pentium III-based and higher)
:Intel Pentium 4 (all)
:Intel Pentium D (all)
:Intel Core Solo/Duo (all)
:Intel Core 2 (all)
:Intel i3/i5/i7 (all)
 
:We apologize for any inconvenience that this may have caused. We appreciate your understanding, and we will do everything we can to ensure that your World of Warcraft experience is enjoyable.
:-Courtesy Blizzard Support Centre
To check if you can fix it in your bios, load into bios and there is a setting for SSE Extensions. Toggle it on, and save to your CMOS. 
If this doesn't work, this means your system isn't going to be supported due to extreme age, and to not expect any assistance if you see this popup.
Category:Technical support