This is typically something you need a hex editor to view properly. Even if you remove the nuls (which I have tried and is very hard since you can't copy and paste NULL into the replace box) it'll still be gibberish.
He's trying to see what it really is nhlkoho, replacing it would accomplish nothing. scatman is most likely right on the money. A hex editor or a resource viewer might do the trick for whatever file you are trying to look at.
You can replace NULL characters using Notepad++ by setting 'Search Mode' to Extended (\n, \r, \t, \0, \x...) Then put the following in the 'Find what' field: \x00 Then don't put anything in the 'Replace with' field.
