I had a similar issue when I switched from Windows 7 to Ubuntu and found Fixing the MBR quite helpful.
Here's the link to it: http://www.tomshardware.com/news/win7-windows-7-mbr,10036.html
Fixing the Master Boot Record (MBR)
Step one: Turn your computer on, booting from either your Windows 7 Installation DVD or Windows 7 System Recovery Disc. Remember, you may need to change the boot order inside your BIOS to have the your DVD drive boot first.
Step two: After the installation or recovery disc loads, if prompted, select your language settings and then continue. If you are using the installation DVD, when prompted by the following screen select Repair your computer.
Step three: The computer will take a moment now to scan itself for any Windows installations, after which you will likely be given a choice to select which installation you wish to repair. Select the appropriate Windows installation from the list and then continue. If by chance a problem is detected in one of your Windows installations at this initial stage, the system may also ask you if it can try to repair the problem automatically. It is up to you if you wish to let the system try to repair itself, but otherwise just select No.
Step four: Once you have reached the System Recovery Options screen, as shown below, you will be faced with a list of choices that can aid you in repairing a damaged Windows 7 operating system. If you wish to try the Startup Repair option first, it is often successful in automatically fixing many different start up issues, but in this article we will be using the Command Prompt option to resolve our problems manually. So, click Command Prompt to continue.
Step five: Now sitting at the command prompt, enter the following command and then press enter:
If successful, you should be greeted with the message The operation completed successfully. That's it! Your Master Boot Record has been repaired.
While the above command does fix the MBR, and sometimes that is enough, there still might be an error with the system partition's boot sector and Boot Configuration Data (BCD). This might occur if you have tried to install another operating system alongside Windows 7, such as Windows XP. To write a new boot sector, try the following command:
If you are still faced with your Windows 7 installation not being detected during start up, or if you wish to include more than one operating system choice to your system's boot list, you can try the following command to rebuild your BCD:
The above command will scan all your disks for other operating systems compatible with Windows 7 and allow you to add them to your system's boot list. If this fails, you may need to backup the old BCD folder* and create a new one in its place with the following commands:
bcdedit /export C:\BCD_Backup
attrib bcd -s -h -r
ren c:\boot\bcd bcd.old