This very well might be a memory leak caused by a WinXP update. I've been researching a very similar situation, but haven't been able to cure mine (nor have I been able to receive much help from people here, unfortunately
One of the things I've read suggested is to stop the windows update service altogether, rename the folder, and then restart the service - this will cause it to re-write all of the files associated with it and hopefully fill the hole.
Start -> Run -> type cmd in the box and click ok to open a Command prompt window
type net stop WuAuServ
That will stop the windows automatic update service.
Type cd %windir%
That takes you to your windows install directory
Type ren SoftwareDistribution SoftwareDistribution_.
This will rename the config for Windows Update
Type net start WuAuServ
Restart your computer
At this point, your Windows Update configuration should be rewritten and should hopefully fix the leak. Unfortunately, this did not work for me, as my system was unable to stop the service, but I'm going to keep on trying other options...