You need drivers wpdusb.sys and WudfRd.sys in your drivers folder. C:Windows/system32/drivers
Place the drivers in there but do not replace a newer version with an old one. Most likely you are missing WudfRd.sys
I had the same problem today. Luckily I went to the machine I know the sansa worked on and copied the drivers to get it working on my new machine. It also is XP Pro. Good luck.
