Any PC that can run x86-64 instructions with full 64-bit memory addressing--though I'd recommend at minimum 8 GB of RAM (RAM is dirt-cheap nowadays). That means any PC that uses the Conroe-core Intel Core 2 Duo or Dual-Core Pentium CPU or the AMD Opteron or Athlon 64 CPU can run Windows 10 pretty well--essentially most PC's built since the fall of 2007.
There should be a bunch of If's added to that statement ;
If the BIOS has support for that version of Windows.
If there are drivers available for legacy hardware in the PC.
If the CPU is powerful enough to handle the Windows Bloat that occurred between the time it was new and now.
The primary issue here is full hardware support for x86-64 CPU instructions and real 64-bit memory addressing if you have 4 or more GB of RAM. I have an old machine running the Intel Dual-Core Pentium CPU (essentially the Intel Core 2 Duo CPU with less on-die memory cache) and with 8 GB of RAM installed; Windows 10 will run fine, if just a touch slow due to CPU limitations.