Are you using XP, Vista or 7? And what sizes are C and D?
If Vista or 7, you could temporarily free up a few GB on C: by disabling hibernation. XP also has hibernate but it is usually smaller.
Running Ccleaner will also free a bit of space.
That should allow you to copy iTunes back.
Seeing as you haven't been using D: partition, there is no point in having it. You might as well just have one big C: partition.
Start run diskmgmt.msc. If C and D are next to each other, you can delete D (when empty) and expand C to fill the Unallocated space. Right click the partition for options.
If Disk Management options are greyed out, use Easeus Partition Master instead.
If C and D are not next to each other, you have to delete D and move the intervening partitions to the right, so that C has Unallocated space to the right of it. Then you can make C bigger.
These problems are much easier if you had an external drive. You can also make sure you have secure backups too. You may lose everything sometime.
Ext HDD 500GB-1TB £50-80