Jim Corenman and Sailmail highly recommend the Tripplite / Keyspan USB to Serial adapters....because they work...
Particularly the Keyspan USA-19HS
Also, be sure you have the correct/latest Airmail software
(version 3.4.034a or later), as this will work with Win7 and Win8....
Further, remember that with 64-bit systems, you'll need the correct SCS drivers..
For the PTC-IIusb and Win-7/x64 (64 bit version) there is the same issue as Vista/x64, the drivers from the SCS website are needed. Go to www.scs-ptc.com, select "software" and scroll down to PTC-IIusb drivers. Those are unsigned drivers so Airmail cannot pre-install them, they need to be done separately- follow the instructions on the SCS website.
Be sure to read, save, and/or print-out the Sailmail Primer....as it answers many questions on its own..
I hope this helps...
s/v Annie Laurie