Slowest PC to support SSE2 is the Pentium 4.
https://en.wikipedia.org/wiki/Pentium_4 But that being said, I also suggest a newer computer and give up on this old computer for Windows 8, 8.1, or 10 and stick with an older OS on it such as XP.
As camerongray stated its probably best to buy a new computer prebuilt/bundled with Windows 8. If you were planning on getting by with a older slower computer and Windows 8, then I am guessing that your computing requirements are low and one of the green computers with modern but slow CPU or APU might be for you such as if this is just for web surfing and e-mail and no games, however if you plan on playing games then a Intel Core i3 or AMD Athlon FX-4300 or better is suggested as minimal. And if you plan on playing graphic intensive games on high settings then your looking at a modern Core i7 or AMD 8-core FX-8350 etc.
Remember that Mobile Processor i7 systems are usually weaker than Desktop i7 systems, so dont just go with the i7 name for Intel Core i7. Do research first and verify that its going to run what you plan to run on it and the GPU ( video card ) also is a good match for your needs.