Insert your bootable Windows Vista CD into your CD drive and boot from the CD. You may have to change your bootup settings in your BIOS settings to make the computer boot from CDs before the hard disk drive.
Then just follow the on-screen instructions. Post back here if you have any questions during the process.
Backup any data you don't want to lose before you reinstall.
Here is a link to instructions for your model (Scroll down to the section called "Hard Drive Recovery" and follow those instructions: