You need to provide your version of Windows to get a good answer. Here's some
tips for Windows 7:
If you have an optical drive
* Create a System repair disk using the Backup and Recovery selection in the
control panel.
* Change your BIOS to boot from CD/DVD first
* Boot the recovery disk
If you have a netbook (more work)
* Download the System repair disk from here:
http://neosmart.net/blog/2009/windows-7-system-repair-discs/
* Get a utility such as UltraIso (free trial available)
* Use this to burn the iso file of the system repair disc to a usb drive
* Change your BIOS to boot from USB first
* Boot the recovery disk
Both methods join here:
* Select the default options
* Wait for Windows 7 to be detected
* From the dialog box select Command Prompt
* Type the following line exactly:
Bootrec.exe /FixMbr
* Wait for success message
* Type exit
* Select restart and change your BIOS settings back