Here is how I do it and it is very simple:

Download and install Filezilla FTP server on your PC, set up a folder for FTP share and set up the password. Copy the files you want to transfer from PC to N800 into this folder on the PC.

Now from your N800 browser, type ftp://your_pc_IP_address like ftp://192.168.1.100 and you should see your files in that FTP folder on the PC. Download and save them to your N800.

But if you have a SD or SDHC card reader for your PC, you could just copy the files on the card directly.