Use disk defragmenter and disk cleanup.
they are located in the system tools menu (windows xp)
Start:all programs:accessories:system tools
also, compress your files, save lots of space:
Right click on C drive (or the drive that your system is running off)
and choose "Compress drive to save disk space" it may take a little
while, but, pending on the total size of your drive, most likely will
take less then 1 hour. Don't be fooled when it says that it will take
1 day to clean up; it will change to 23 hours and keep on going down
quickly.
copying over files is easy... you can choose the "Copy to" or "Move
to" selection on the sidebar in any folder, or you can open up D
drive, open up the folder where the files are, select the files, right
click, choose copy, go to D and click paste.
Hope this helps!