If you run Ccleaner and tick the Hotfix Uninstallers box in the Cleaner section and then click Analyse button it will give you an idea of how much the uninstallers for the Windows Updates are using. You can usually safely get rid of the hotfix uninstallers (I have NEVER used one). But it is safeft to have a backup (e.g. Acronis) before you do anything drastic.
Do you have two identically sized disks or one disk partitioned in to two simialr sized partitions? In any event, what do you store on each (assuming that, at least, OS and pgms are on C)?
With Cleaner selected on the far left (this is the default that Ccleaner opens up with anyhow), Select the Windows tab (again the default). If you look down the list of areas to clean, under the Advanced sub-section you should see Hotfix Uninstallers. Although the Advanced sub-section may appear greyed out at first, if you click on the Hotfix Uninstallers check box it will be selected and the Advanced sub-section should then not be greyed out.
I suggest that you then click Analyse (this doesn't delete anything). You will then be able to see how much space might be saved by "Run Cleaner" with this option selected. Please note that if you do not uncheck Hotfix Uninstallers it will remain selected next time (and every time) you Run Cleaner and will then automatically clean them up anyhow.
As I said before, you are best off making a backup using something like Acronis True Image first. In the short term you may well be able to make an image of C: on to D:
If all you have on C is OS and pgms, 69GB should be ample. Can you tell us how mauch free space you have on each drive?
Also, if you open up Windows Explorer and look at the top level folders in C: (e.g. Documents and Settings, Program Files, Windows), right click on each in turn and select Properties. This will then tell you how much storage (Size on Disk) is in use for each path. Post the information back here.