major missing hard drive space issue

I have a laptop computer running vista ultimate 64 bit. When you go to my computer and look at the c drive it says the total space of it is 104 gb (8gb are partitioned making it a 120Gb hdd). It also says that 92gb are used on the c drive. When you go into the c drive and select all the folders it gives a value of 42gb used which gives 50gb unaccounted for. I have gone through the steps to gain access to the system volume folder. I used windirstat and it shows 42gb used and 50gb as unknown. I think windirstat looks at the sizes given then measures all the files and folders so it sees 50gb unaccounted for. I have run a pcpitstop report on the computer and the hdd score was god awful (single digits). It always seems like the hdd is going since the installation of Adobe Lightroom. I have another computer with pictures on it that it sync/indexes with when i connect them over the network once and awhile but that is not often. I have also gone through the system restore cleaner already. Any ideas on what could be grabbing 50gb?

it's a huge paging file



probably a good reason not to run 64-bit vista with such a small hard drive

the paging file is 4gb, i can see that in windirstat

There is no backup going on I have gone and disabled the system restore and cleaned the restore out. 50gb still unaccounted for. On another site someone suggested a rootkit scanner but I would need one that works on vista 64bit, know of any?