if your boot block got corrupted (virus, or installation which went wrong), you may want to try to repair it by using the Recovery Console of Windows XP.
To do so, boot on the Windows XP CD and when asked, choose to repair the Windows XP installation by pushing R.
This will lead you to the Recovery Console. You will be asked for the installation to repair (ex : 1: C:\Windows), and then to enter the administrator’s password.
Then you can try to do a fixmbr (which is equivalent to the old fdisk /mbr) or a bootblock which will install a new boot block.
Be sure to understand what you do ! You might as well spoil your last chance to recover your installation with those commands !