You'll need a CPU supporting the Intel SSE Instructions (P3, P4 or an AMD XP CPU) with at least 1.6 Ghz.
Memory wise, you should at least have 256 MB of RAM in your system.
As for the OS, you'll need Windows XP or 2003 (the emu will NOT work under Windows 9x or ME, 2000 may boot certain ISOs but we won't go into detail on this now!) together with DirectX 9.0b and a graphic card supporting it either from Nvidia or ATI together with their latest drivers.