An alternative to the packaged Dex Drive software:
http://www.neillcorlett.com/dexter/
I will try this out on my serial Dex Drive later to see if it works.
A while back I remember a program called something like User Port (which I cannot find now btw) which allowed unprotected port access to serial, parallel, and MIDI/Gameports on Windows XP. It installed a driver on your system, then after reboot allowed you to tell it what ports to allow full access to (XP seems to have added some strange handling for the older port types, disabling many programs and old printer drivers). It worked for some programs (including my N64 Dex Drive) but not others (had to upgrade my serial GraphLink cable to a USB one). Dunno what happened to this program tho... I looked for it on Google and I will look for it on my PC as it is prolly buried deep in the heaps of "Zips that came off of my desktop" type folders...
EDIT: I just tried out the Dexter software I linked to at the top of the post, and it works great. Also works with ePSXe's mcr format, as well as many other common formats for both full memcards and individual saves. Copied my FF8 save from ePSXe memcard to my Dex Drive, booted FF8 up, and it loaded fine off of the card. I would highly suggest this program for anyone who uses ePSXe and a serial PSX Dex Drive.