I, too am having this same problem. MPCstar starts excruciatingly slow. It takes about 25 seconds to start and about the same time to load a video of any format. I didn't have this problem with WXP. It seems to have started with W&. I'm running W7 Ultimate, SP1 on an AMD Athalon IIx4 640 processor at 3 GHz with 4 GB RAM in 64 bit mode and ASUSTeK chipset.
I love MPCStar and have been using it for years. This problem is so frustrating, though that I've been relying on AVS for my viewer with it's limitations vs. MPCstar. I'm using TigerPlayer 188.8.131.52.