My laptop will not boot up. What's wrong with it? and how can it be fixed?

Use the windows CD if you have it.

Start the computer and enter the BIOS. Change the 'boot priority' so that the CD drive comes before the HDD. Save your settings and restart.
Insert the windows CD upon startup and press a button when prompted ("press any key to boot from CD"). When the disc goes to install a new copy of windows, it should spot your existing installation, and offer you the chance to repair it. Follow the instructions and see how that goes.

Failing that, enter the windows recovery console and repair the Master Boot Record. If that still doesn't work, your HDD may have kicked the bucket. Can you hear it spinning up?

Assuming the drive is powering-up, you could remove it and use another computer to try and acess the disk as a secondary drive. This would allow you to remove all data from your drive before totally wiping it and trying again from the start. Anything beyond this should really be left to an expert.