Sometimes my computer will freeze and then a it will say that there is no operating system. once I turn it off and back on it will work normally, what could be wrong.

"Operating Systen Not Found" are the exact words that it uses, it also happended when I tried to defragment it. can someone tell me what the problem is?

Boot the machine with your XP CD.

If/when you're told to "Press any key to boot from CD", hit the Space bar.

At the 'Welcome to Setup' screen, press r for Repair.

When the machine is done loading, type in;

Chkdsk /f

When it's fininshed, try booting normally.

If it still won't boot, repeat the above, this time type in;

Fixmbr