defragmenting hard disk

My computer works very slow so I started to defrag the C: drive but when it reached 99% it won’t do anything else so I have to restart the computer but it never started again.
I only see a sentence flashing but I can’t read it because it flashs toofast
I’ve made a restore diskette but it start and you see the logo of win XP then it keeps flashing again with one sentence. I have an win XP and office XP IBM netvista computer.
what can I do?