Convolver is an open source, high performance Windows application for applying finite impulse response (FIR) filters to multi-channel digital audio in the form of a real time DSP audio plug-in (DMO) for Windows Media Player.
The install package also contains two DirectX/DirectShow filter versions — ConvolverWrapper or the equivalent ConvolverFilter — for Adobe Audition and real time applications such as Zoom Player Pro, TheaterTek 2.2, Windows Media Player Classic, J.River Media Center (when used with the DirectX host plug-in), or Console.
See the setup walkthrough for ZoomPlayer after reading this page for further details. In many cases the DMO will also work.
Performance is excellent, possibly the best available under Windows, and subject continual improvement. A stereo 65536-tap filter, the largest that makes sense when applied to a 44.1kHz source, executes at 40 times real time, representing a 3% cpu hit, on a 3.4GHz Pentium 4.
Even on a 300MHz Pentium II, the reported cpu hit is about 30% when convolving with such a filter. So your old machine can be put to good use. Mixing channels results in some slowdown (six 65536-tap filters will consume less than 10% cpu on a 3.4GHz Pentium 4).