How can I fish out large files that are hidden?

I have a user that I just issued a Lenovo Laptop with a 40Gb hard drive. It’s been a week, and now he comes back and tells me that he only has 9GB left. What is going on here? i checked his PST’s and his program files. I also checked his applications in add/remove to see if there were any rogue applications installed. It was clean. Anyone have any ideas?