Do you have a Norton Protected Recycle bin?
I've used a free app called i.disk that lists the space used in all the folders, quite handy for finding where space has gone. Get it here :- http://www.memecode.com/
If these files are AVI then VirtualDub is a good free joiner , among other things. The key here is to use Direct Stream Copy when resaving the file. This avoids any recompressing etc & would give you a file size equal to the sum of the 2 individual files.