Try opening Windows Media Player and checking the version, just to be safe. Click on Help > About and be sure that you have WiMP 11.
Windows XP is bundled with WiMP 9, and the update may have zorched your previous MTP-capable installation.
Good thing you have installed the MTP Porting Kit. MTP is actually a virtual mode that treats your Sansa as a media player rather than a simple MSC flash drive. This makes the mode more complex. One of the differences is the “wall” between files visible in MSC versus MTP, as you’ve discovered. Files transferred in one mode are not available (on the PC side) when in the alternate mode.
Be sure to set your player back to Auto Detect or MTP before going further. In fact, for diagnosis, select MTP to ensure the Sansa is trying to “speak” MTP while we delve into it.
Try entering the following commandas described in this post. It helps in Code 10 issues.
Reboot the PC.
Once it’s done booting, go ahead and plug in your Fuze, and press [Windows Key] + E, or go to My Computer to open a Windows Explorer window. We want to see if it pops up under other , meaning that it’s in MTP mode. If it is listed as a media player, you’re back in business.
If not, open the Device Manager and look for the Sansa in the USB hub. It will be listed with a wee yellow triangle of doom in many cases. If so, right click on it and select uninstall. Disconnect, reboot again, and plug in once the system is stable.
Your Sansa should be happy in MTP mode.