I'm running Win 7 32 bit. It's possible there's a 32/64 bit issue with the URC software as I didn't have any problems at all. Assuming that isn't the case, what COM port are you using in the software? Check the Windows Device Manager and see which port it's assigned to your USB to serial adapter and make sure that's the one the software is using. Go to Device Manager and expand the Ports sub-header. You should see your adapter listed there with a COM 3 or something at the end of it. This can change between uses if you add/remove USB devices or plug it in to a different USB port so this should always be your first check if it's not working.

Also, verify you're using the correct drivers. I know you said it installed the Prolific driver but you didn't mention whether that was the right driver or not. You'll want to make sure that your adapter is compatible for 64 bit, too. Many of them aren't; adapters that have been working for years on 32 bit just aren't given 64 bit compatible drivers. Don't forget to check the manufacturer's web site for updated drivers if Windows isn't installing the adapter correctly.

We have to use USB to serial adapters frequently at my job. The most common errors we see people making are not installing drivers, installing the wrong ones, or using the wrong COM port (often even after installing the correct drivers).