Here is a link to instructions on how to hopefully fix your problem. Just make sure to follow the instructions exactly. There are two things the page does not mention that I will try and explain.
1. You will need your windows 7 dvd. If you do not have this, you can create a windows 7 repair disc to use. Hit start, type repair, then choose "create a system repair disc". It will walk you through the process of making this disc. Once created use that disc to boot off of and follow the instructions in the link provided.
2. Booting off of a disc. You can try booting off of the disc first to see if your BIOS is already set to do this. But if when you restart your computer it loads up your normal windows installation you will need to enter your BIOS to change your "boot order" so that the cd/dvd drive boots first. In order to enter you BIOS you must hit a certain key before windows loads. The "key" is different for different motherboards. It will usually tell you briefly at the bottom of the screen when you turn on your computer. Some common ones are F1, F2, Esc, Delete, and F10. Once inside the BIOS you will have to navigate around looking for "boot order" and change the first thing to cd/dvd drive.
Just set aside some time, and be patient. It's much easier then it sounds.