>I want to locate a file in my drive, I used fullpath command but it is always

Fullpath() doesn't do any validation on the file name; it returns a path to what would be the file name (if it doesn't exist) it's most useful (?) for getting things like ?fullpath("..\Data")

If you want to locate a file; try

if file("MyFile.dbf")
? locfile("MyFile.dbf")
else
? "MyFile doesn't exist in the path"
endif

that will return the full path to the FIRST copy of MyFile.dbf it finds.
Cheers
-josh