This quick post will definitely help you if you have messed up with ypur MBR i.e. Master Boot Record.First of let me tell you what MBR is.
MBR or Master Boot record keeps the information about all the boot softwares ,or in laymens language we can say list of all operating systems. You mess up with your MBR when you use more than one operating system as n case you may be using a combination of Windows and Linux.
It may be irritating sometimes as you must have installed Linux and cant boot it or in another case you must have installed linux and cant boot XP.
Now you have two options, either reformat the disk and reinstall the OS in a sequential manner or just use this software.
You can use the disk editor programmes provided by the comapnies also such as Seagate provides seagate disk editor
The other technique is using grub or lilo which can fix your MBR.
Hope it may have helped you