If your install has failed you probably have 2 main options to get everything up and running again.
1 . Install ubuntu again
To reinstall ubuntu put your live disc in and go through the setup again, when you get the the screen asking if you want to install alongside windows, replace windows or configure manually select configure manually. Then identify the new partition that has been created with your partial linux files on, format it and select that as your install location. Then continue with your installation normally. Ubuntu will then start the installation over again and when completed will set up your GRUB with both operating systems listed when you boot.
2 . Restore the Windows 7 boot and worry about ubuntu later
Insert your Windows 7 Disc
Press a key when you are prompted.
Select a language, a time, a currency, a keyboard or an input method, and then click Next.
Click Repair your computer.
Click the operating system that you want to repair, and then click Next.
In the System Recovery Options dialog box, click Command Prompt.
Type Bootrec.exe, and then press ENTER. This will give you a guide on how to use the tool which you can also find here http://support.microsoft.com/kb/927392. But what you need to do is to fix the boot by typing
Personally I would recommend the 1st method as if you want ubuntu in the future you will have to do it at some point anyway so there is no point in fixing Windows 7 just to replace it with ubuntu again.