Check the contents on the .inf file...
I had a problem with a 2 RS232 PCMCIA card not installing - it used to install perfectly. It turned out the following line was wrong in the .inf file.
ServiceBinary = %12%\oxser.sys
The file this refers to was in capitals in the installation folder. I wouldn't have thought it would matter but I changed the file name and I'm back up and running the card.