|Try f8 as soon as you turn the computer on and choose last working configuration|
If no joy, you need your xp cd ....boot off it...do not except the first repair option.......the option comes around twice.
For a concise repair tutorial google...it.
The problem is caused by a missing or corrupt file.
You will lose any updates and any special drivers but retain everything else.
Hope this helps
Being a slow typist an answer was posted before this one.