C.Drive full D.Drive loads of space?

My Windows XP C.drive (10.0GB) is almost full,my D.drive(114.0GB) is almost empty,C drive seems to be taken up by Windows,I have only a few programmes of my own on C and deleted Photoshop etc,from C but can't download anything as I'm not always asked where I want the prog sent,can I move all or most of C drive to D drive,or set a Default to divert all downloads to D drive.I'm an old git but my windows 98' had loads of quality progs on it, now it seems to take up all the space for it's own good,don't have Office,Word,lots of stuff I used to have with even less memory,can't even defrag properly,I don't need a lot but it seems a case of the tail wagging the dog.Please! how can I shift everything over to the bigger drive.P.S try to keep it simple,not very IT.Cheers.