Locks up computer everytime

I just started using Defraggler. I tried it about five times and it locked up my computer every time between 69-72 percent defragged.

I tried disabling System Restore and using it in safe mode, same thing. The only way to get out is to shut down the computer. It just freezes, no error messages or anything. The rotating arrow continues rotating, but everything else freezes, can't pause or stop the defrag, can't close the program, total freeze.

Acer Aspire One netbook

71% available space on drive

Windows XP Pro SP3

Thank you,

Gary