here is what I would do... get the properties on each folder in the root of C:\
ie, Documents and settings, Windows, Program files... And C: too.
What is each one? Then you can narrow down what it's holding. Also, look at the systemrestore settings. Right click my computer, Properties, System Restore. What is the capacity set at? shrinkning that to under 10gbs is good. If it's set to 10%, then you can be allowcating 25+ GBS to it. Also, what is your page file set at?
Another thing to try is a disk size scanner. Something like this
it will show you a visual representation of the space you have in use and also freespace, so you might need to uncheck the "Show free space" option.
Something else to do is run a checkDisk. This might clear invalid entries in the File space table...