I really screwed up my computer and I'm pretty sure a clean install is my only option. I used a CD to install it. How to I uninstall and reinstall ubuntu? Acer Aspire 3690
Don't concern yourself with uninstalling, simply boot the CD, read and follow the prompts. If you like, you can use the same disk partitions, or redefine them. To be sure that there's nothing left, elect to "Format" the partitions.
Better luck next time!
You could also boot the CD, select "Try Ubuntu without installing", mount the harddisk, and copy your important data from it to another disk, USB key, or whatever.