After messing up my Win 2000 desktop I took it to the local repair shop where the incompetent owner restored it to life, but managed to lose about 50 GB of hard disk space. Yes lose it. I've searched meticulously for a file that big, without success. What can have happened to such a big chunk of HD Space. I've tried to start defragging but get told that the C: Drive doesn't need defragging! Total C: Drive is 180 GB. I still have 85 GB remaining free, and can account for another 35 GB of existing Programmes and Files- a total of 120 GB.
Any Suggestions ?

most likely caused by the way he formatted the drive go the computer management ad disk management anf it should show as unallocated or something like that ,you should be able to right click on it to set it up and format it get it back as a second partition and use it for storage .
go to start/ run and type or copy this into run compmgmt.msc then go to disk management in the left pane

If I'm reading this correctly the partition is 180GB (if that's the case you should follow Capers instructions as well since no drive size I know of would partition to 180GB) and you don't know where the extra 60GB of used space is?

I was discussing a similar issue with a colleague just a few hours ago. My first guess would be that there is some kind of back-up folder (or file) on the drive that you're not aware of. If you can't find that what I usually do to track down space usage is go to my root directory (usually C: in my computer) and use right-click & properties to check the size of folders. WINDOWS (I think it's WINNT for you), Program Files, and Documents and Settings are the most likely culprits. Once you've found out what folder is eating the space you can follow the same steps down the directory structure to it's root.

I also ran across this little utility in my discussion earlier:
http://foldersize.sourceforge.net/
It allows you to navigate the folder directory while displaying the size of each folder, the same way windows explorer does for files when you're using the "detailed" view. It could help you in this.

Now, should you think that your partition is smaller than it should be you should follow capers steps to utilize the un-partitioned space (personally I'd suggest using a partition management tool to resize the partition rather than creating a second, but that's a little more advanced and is a matter of preference.

sorry, i read it wrong in first post !1

Happens to the best of us, but I think you might be right too, Would a 200GB drive format down to exactly 180?