The best method of directing file types to point at a desired program...
1. Open any folder you have readily accessible.
Click Tools--> Then Folder options
2. in the next window, select the "file types" tab, find the file extension you are looking to have Paint.net open as a default. (jpg, gif, etc...) and click CHANGE.
3. The next window will be a list of recommended programs. If you don't see paint.net click the Browse button and locate it. It should be listed under your program files. Select the Paint.net program itself and then click ok. You will need to do this for each individual file type you are wanting to associate with Paint.net. But it will make your life a little easier and you Won't have to delete other programs as mentioned in the previous post.
Edit (in addition)...
If the Jasc and Corel Products came already installed on your computer when you got it, you should be able to simply register the products and they are yours. This is COMMON with a DELL if that is what you have.