Windows uses something called a registry, that keeps track of programs. Moving programs by simply copying and pasting the files won't work because the registry won't know you've moved them. You can uninstall your programs from the C: drive and reinstall them to another drive, but I wouldn't recommend installing programs to an external drive.
My Vaio PC keeps reminding me that C drive is running out of space. I have already transferred all documents, pix, etc. to external drive G and have defragged, etc. but reminders keep popping up. Pie chart with available space on C drive shows a sliver of free space. My external G drive is huge. Can I place all programs on it?