The C:\ partition is for Windows XP, programs and software.
The D:\ partition should be used for your personal files, documents, music etc, SonicStage and DVGatePlus use this for temporary and permanent storage.
The reason why you should use your D drive is because if you ever need to reinstall from the DVD disk/CD set/Recovery Partition (delete as appropriate) then by doing a standard recovery the C dirve is formatted to factory default and the D drive with your data on it is still intact...
I hope this helps!
