twiggs solution would work if you have one hard drive with two partitions. If you have 2 hard drives then it won't work.
No one has asked the question though, why is your C-drive full.
One reason is programs install by default to the C-drive. This is sensible in most cases but if you have a particularly large program, like Autoroute, then it takes up about 1Gb and could happliy sit on the D-drive instead.
Another reason is loads of dead files such as ZoneAlarm logs all ending in .TMP. You can search for these and delete them. I had a glitch with Symantec and had over 16000 dead files. Talk about a full HDD.
Then you may have stacks on Uninstall files in your Windows Folder. These will be in folders such as 'kb96457665' and are installed when Microsoft does its weekly update. They enable you to roll back from an update. After a short period of a month or so you will probably not need to uninstall so these are simply dead space. I have just cleared a friend's computer of these folders for 2004-2007!
Then you may find the Prefetch folder is large too. A file might only be 300 b long but still occupies 4000 b of space.
Then your Documents folder may be stuffed with digital images. Create a new Documents folder on the D-drive and then redirect your documents etc to the D-drive. This is best practice as it separates the application files, that don't change often, from data files that do. It also simplifies finding data and backing up your data.
Then you could compress the drive. Open My Computer, right click on C-drive, select properties, tick Compress drive to save disk space.