Also how big is your SD card. XP does not have native support for SDHC which is for cards over 4GB. You can test this on multiple computers, but if they are all XP it won't work on any of them. Find the XP hotfix or preferably a Vista/Windows 7 box
http://support.microsoft.com/kb/934428
Let's assume you have tried a Vista/Win7. We're going by the odd assumption that it works on the DS, your only option is probably to use DSFTP and suck the files off. Note a couple of things
1) The wifi ds library sucks terribly. So be prepared for disconnects and other randomness
2) It could be very slow depending on various factors. You might want to prioritise what you pull off. Since you seem to be concerned about you save file pull those off before deciding if your ROMS are worth it (I assume you have backups of those)
DO NOT FORMAT THE SD CARD YET
Take these save files and test them on another SD card you know is good. Again this will verify that the FTP transfer worked ok and the files are ok. Once you have tested all your saves, reformat the old SD card and start over.
Note that your situation is very odd. I am not even sure if the data you will get off the FTP transfer won't be horribly corrupted in some way even if the transfer succeeds.
If you tried these without success then you're pretty much SOL. Refer to the Panasonic Formatter and start over.