How do i

Put your system restore disk in your cd rom then restart the computer. It will give options for repair or restore.

You need a Windows installation CD together with the Product key, which is printed on the lable on the bottom of the laptop.
The Product key is for a particular version of Windows and that is also printed on the label.

I suggest you download all the device drivers (eg. audio, video, modem, network card etc. and they need to be for your version of Windows drivers) off the Sony WEB site before you erase/wipe your hard disk.

You then place the CD in the laptop and boot up with the CD (on some laptops by pressing F12 it allows you to select an alternate device to boot up - select the CD) If this option is not available to you, then goto the BIOS setting and change the boot order and select the CD as the first boot device, you can change it back to the hard disk after you install Windows.
When you boot up from the CD, you will begiven the option to delter the existing partition and start afresh, or you can over write the existing files. Just follow the screen instructions.

When Windows is installed you will find Windows has not installed some or all of the device drivers and you will nedd to install these downloaded drivers.
You will need to install the latest service packs (for XP it is SP3, for Vista it is SP1)

It would help if you had a knowledgeable friend to assist you if you have never installed Widows before.