Hi guys!
To format your HDD and install WinXP you should put your WinXP disc in CD/DVD player and restart machine.Then get in to BIOS and change boot priority for CD/DVD device to be first on booting process.
To get in to the BIOS find which key you should press when PC starts (delete , F2 or depending which BIOS you have) if you can tell the brand of your lapy I can help you to find the key.
After the PC starts just press any key to boot from disc and then follow the prompts.Basicaly click enter , select partition to install WinXP , format using NTFS quick.More info here
http://support.microsoft.com/kb/316941 Before that back up your documents, music and pictures , e-mail addresses.To back up e-mails read from this link
http://support.microsoft.com/kb/270670 
Hope it will be OK