This list was created for the purpose of documenting final releases of programs, drivers, and hardware to support for computer processors that lack SSE2 instructions and higher, and this list is designed for people who have old computers such as the Intel Pentium III which lacks SSE2. Please DO NOT email me stuff that are piracy, I won’t add it on this list. I will try to update the list if there's more or whatever changes. The last Microsoft Windows operating system that works without any modifications and doesn't require SSE2 is Windows 7 with Service Pack 1, but Windows XP is more suited for old computers since new updates in Windows 7 wouldn’t work properly like the latest .net Framework.
Note: Just because an application doesn't require SSE2 doesn't mean that it doesn't require MMX or SSE. Therefore it may still not work on even older processors such as 486 or Pentium II processor. Also, please note any software that is ongoing many stop supporting the old non-sse2 processors in the future. I highly recommend you to bookmark this page if you want to make use of this list.
UNS = Unofficially supported, but works
ONG = On going support; supports non-SSE2 cpus fine.
Steam (ONG) - Steam Browser is disabled meaning that you can't use the steam store or use anything that uses the Steam Browser. By 2019, steam will no longer work on non-SSE2 systems and drop support for Windows XP and Vista.
If you want to use Discord, then you must use the web version since the desktop application requires SSE2 and at least Windows 7.
Half-Life 1 Steam version (ONG) Works perfectly and no issues whatsoever. includes the following list too
Orange Box (No DX6 or DX7 support, only DX8 and DX9):
 1.7.13 and higher require a processor with SSE2 instructions