This may help: You MUST copy the “libmysql.dll” file from you “\PHP” folder to the “Windows\System32” for it to work.
This only appears to be the case with PHP 5.x.x
I couldn’t get WordPress on my localmachine to work until I changed back to PHP 4.4.7, which is the same as my hosting company uses, and it ran fine without the libmysql.dll in the system32 folder.