General

@BobR: I've done the 'Add or Remove Programs', I'll try the rest when I get there, thank you very much!
Edit: And I'll stay away from the Windows folder.

[quote=Orange]@greenelf: I don't really want to do a backup, it's just that the pc says the C drive has '0% free space' and the windows folder was the biggest, so I wanted to copy that to the D drive. Her documents folder has already been moved to the D drive.
So is this possible? Or would it just be a real hassle to make everything work again?[/quote]
Don't mess with the Windows folder. That can really mess things up.

First thing- open "Add or Remove Programs" in the Control Panel then go down the list and uninstall anything that's not used.
People tend to install all kinds of crap, use them one time and never go back and use them again, but they sit there on the hard drive just taking up space.
Uninstall anything that's not important.

Next thing- open "My Computer" then RIGHT-click on Drive C: and select "Properties" from the pop-up menu.
Click the "Tools" tab at the top of the Properties window.
Click the "Check Now" button under "Error Checking".
Check the TOP box on the little popup window then click "Start"
It will tell you it can't run the scan because Windows is using files and ask you if you want to schedule the scan for when Windows restarts. Click "Yes".
Restart the computer.
This will scan the hard drive looking for errors and try to fix any errors it can find. This can free up wasted disk space by finding and marking "lost clusters".

Next open My Computer and RIGHT-click the C: drive again and select "Properties" again.
This time click the "Disk Cleanup" button under the big "pie chart".
After it scans the disk it will pop up a window with a lot of check boxes on it. Check all the boxes, then click "OK"
This will clean up the disk drive of all the old useless stuff that has built up over the years, empty the Recycle Bin and delete any lost clusters the Error Check found.

These procedures may clean up a fairly considerable chunk of hard drive space depending on how messy the drive was to begin with.

There are a few other Windows settings you can change to make Windows use less of the hard drive for itself, but the most space will be freed up by just moving stuff other than the Windows folder to the other drive.

@greenelf: I don't really want to do a backup, it's just that the pc says the C drive has '0% free space' and the windows folder was the biggest, so I wanted to copy that to the D drive. Her documents folder has already been moved to the D drive.
So is this possible? Or would it just be a real hassle to make everything work again?

[quote=YourExcellency]you see....well, i mean...it's like.....(wait a minute pls)[/quote]
[quote=sparkshooter]I was expecting a...different...result.[/quote]
[quote=Zulucker]Well erm.... (trying to think of what to say)[/quote]
Seriously? Give a real answer, or don't post at all.
On topic: Your windows folder contains [i]all[/i] sorts of stuff, ranging from Error Logs, to important programs, like your registry editor. However, none of your personal documents are actually stored there! Go to your "Users" folder (Usually C:Users). Find the Aunt's username in the folder list, and open that up. You'll see a bunch of folders, like Desktop, Documents, Pictures, and Music. [i]That's[/i] the stuff you want to move to your external harddrive (make sure to COPY it, not move it). If you want to free up some space on your Aunt's computer, run some tools like [url=http://www.piriform.com/CCLEANER]ccleaner[/url] (which will delete things like your app cache) and [url=http://www.revouninstaller.com/]Revo Uninstaller[/url] (which you can use to get rid of random programs).

tldr; Backup your "Users" folder, [b]not[/b] your Windows folder.

I was expecting a...different...result.

Well erm.... (trying to think of what to say)