We had a problem like that with one of the laptops here at work, and after doing lots of testing, reinstalling windows an even trying with a PCMCIA card with USB ports, it turned out it was the mainboard which had a problem (in this case the problem was caused by an "alternative brand" power adaptor which was replacing the original, which fried some parts of it).

So, after all you've done, it seems it might be a hardware problem.