Which PC OS are you using. There are some caveats in some of Microsofts products...

Sometimes it helps to manually unload the USB driver
There are some workarounds described in the lower part of the windows section of the wiki article on flashing.

Usually there are better results when using linux when flashing You could even use some Debian or Ubuntu based Live-CD. Like this great distro from some italian folks http://semplice-linux.sourceforge.net/