I have a desktop PC that has two hard drives. I don't have enough room on one of them and when I try to save stuff on my computer it says there is not enough disk space on my C: drive, but I want it to save to my D: drive. Can someone tell me how I switch this?

Use "Save As" rather than Save. When you click Save As, a box opens where you can navigate to save it to the D drive.

If your C drive is that full, you should get some stuff out of there ASAP.

I would move my big folders [ie media, audio] over to d-drive -- that would free up a lot of space on c-drive.
To move all the files from a folder
1st make a empty folder in your d-drive [lets say my music]
2nd Open the my music folder,go to edit , hit select all, go down the menu on the left and you will see move the selected items- click it and navigate to the new folder.
Individual and groups of files/folders can be moved using variations of this.
It would also be a good idea to back up all important files to d-drive
All the above is assuming that your on xp-home also -- other versions of windows are different
Any time your posting about computer problems always let folks know what kind of operating system you have

The default save location for Office and many other applications is My Documents. You can move your entire My Documents folder.

First, you'll want to create a folder for your documents on the D:. Open My Computer, then double-click on D:. On the top menu, go to File>New>Folder. Call this new folder "My Documents." Next, right-click on the My Documents shortcut on your desktop or in the start menu (NOT the folder you just created), select Properties, click on Move, then expand My Computer, then select the D: drive, then the My Documents folder you created. Click OK, and you may be prompted to move you existing files to the new location. Select Yes.

This will likely move data off the C: drive, and make the D: the default location for documents on most applications.

Most programs allow you to set a default folder for saving data. Check the Help screens for how to set that.