My laptop turns slower!! What is happening??!

Oh My Gosh, a few days ago, my brother found out that the Local Disk(C:) had turns almost completely full, with the bar showed in red color. So he started to delete some unwanted and useless program permanently, and startled to see a folder named by "Windows.old", so he also, deleted the folder permanently. But the thing is, now every time I'm trying to open the Local Disk (C:) file, it would come out like 'Location is not Available, C:\ is not accessible, and Access is denied'.. What on earth is that? Why could this thing happened? Why? Whyyyyyyyyyyyyyyy??????????