I've got an Acer aspire 1683 (win xp) and I can not get the C drive to defragment. I have 52% free memory. I've done a disk cleanup, emptied my temp internet cache/folder, emptied wastebin and so on. I'm getting as far as Defragmenting... 1% and that's it.
I was able to defrag the laptop's other (D) drive without difficulty.