WinXP Single Partition Question about Size of the Root Directory

[drdolittle]

I have been learning how to use ATI 10 on 2 home machines, and noticed that the contents of the C drive on my slower machine is huge, namely 24.5G (select the C drive and check properties), compared to the size of all its contents, which is 10.1G (select all the items contained in C and check properties). The faster machine is much more reasonable at 16.5G compared to 13.7G. I am running WinXPpro on both machines in a single partition, and the faster machine has MS Office installed. The registry size in them is 64M and 84M, respectively.

Does anyone know what could cause this disparity and maybe how to reduce its backup storage requirements?

Thanks for your thoughts.