A couple of our customers have recently experienced some issues with accessing drivers for Windows 7. We've found that the following download will help: http://www.cooldrives.com/lib/cooldrives/vista-prolific-chip-driver-for-serial-adapters.zip. Although it refers to Vista, we have found that this driver does in fact work for Windows 7.
Of course we can't guarantee that a particular driver will work for everyone - we're simply going by our own experiences and those of our customers.
EDIT - this driver is now on our website under USB Drivers