Would you really need to remove the DRM from a Borders downloaded book? Is that just if you want to mess with the formats (as I know a lot of technically advanced users like to do)? I thought Borders/Kobo allow you to download in ePub format. So from that point you can load the file into Calibre, and then sync it to the PRS-300.
I use Windows Vista and if the file format is compatable to my PRS-300, I can also right click on the file and choose 'send to device' - without using Sony or Calibre.