Help! Conroe/DP35DP at half-speed

HELP! New system running at half speed!

Intel DP35DP motherboard
E6750 Conroe Core 2 Duo
G.Skill paired 2x2Gb PC2-6400
GeForce 8400GS (512Mb)
Samsung SATA HD501LJ Hard Drive
WinXP sp2

Video and MP3 encoding goes slower than the Dell P3 500Mhz which it has just replaced. Photoshop redraws are even SLOWER. No errors on POST, but system has been just a bit more crashy than I would expect since I put it together. Mostly just SLOW.

Passmark Overall Benchmark for this processor averages 1500 and I'm barely getting over 700 -- Video Benchmarks in particular are less than ten percent of average speed, Memory performance is comparatively low but not alarming, disk read/writes about one third of average speed. Overall, the benchmark says I'm about the same speed as a P4, with video slower than a P3 500. I don't put much stock in benchmarking, but these figures are way out; and are confirmed by my experiences in photoshop and encoding.

I think something is rather seriously wrong. Where do I even BEGIN finding out what that is? Can anybody recommend some diagnostic software that will actually trace a problem and not just benchmark how comparatively slow I am?

Thanks, rob