Hmmm, I may have a similar setup as yours. Perhaps this might give you a quick clue:
XP SP3, FFdshow (build 1946), ReClock, EVR and HD2600 XT.
I have no problems with DVD playback and AR with MC these days. It's quite stunning, really. My playback monitor is an LG 37" 1080P @ 24Hz.
I'm using the "MT" version of avisyth (2.5.7) and here is my script in FFdshow:
SetMemoryMax(1024)
MT("LimitedSharpenFaster(ss_x=1.4, ss_y=1.4, Smode=3, strength=175, overshoot=0, soft=60)",3)
ConvertToRGB24()
chromaresample="Lanczos"
with "Ignore Pulldown" and Buffers at 10,10
FFdshow Resize is set to "Specify vert and horiz" - 1920x1080, No Aspect Correction and Process Pixel AR internally, Lanzcos 2-tap, 0-luma
FFdshow output is RGB (only option check with HQ conversion checked too).
Are you using the latest FFdshow?
In what order is your FFdshow "tree". I have AviSynth above Resize.
Is your video card set to a native rez of your monitor?
HTH.