It could simply be that all the data is spread out over your C: partition, and nothing more sinister than that. So long as you actually have all the space free on the drive I wouldn't have thought that the Ubuntu installation would cause a problem.
One thing I have done to help with this has been to use a program like MyDefrag to move the data on the drive.
I believe that MyDefrag has an option to consolidate data on a drive and thus move it towards the beginning of a drive. I had a drive that was 1TB in size and needed to shrink it down to about 200GB, before using MyDefrag I was in a similar position to you, afterwards I got it down to what I was expecting.
Here's an image, try the "Consolidate free space" option. You will need administrator rights in order to run MyDefrag.
Another option might be to run the Disk Cleanup Wizard to delete any temporary or unneeded files, in Vista it can also clear the System Restore files which may be free up some of the "locked" space at the cost of not being able to roll back to a previous system state, and I believe it can also delete the hibernation file which may free up some space. See the page I linked for some details of what it can do to help.