I'm not an expert, however the easiest way to default your os to load is to get to the System Properties window (control panel or My Computer, Properties). There in the Advanced tab there is Startup & Recovery Settings button. There you can default your installation & timeout, or edit the file to remove the incomplete installation from the menu.
The next part you may want to validate with someone else, because I would edit the settings to remove the incomplete installation from the startup list, then simply delete the folders the installation refered to. There may be additional junk in the Documents & Settings folder, but it wouldn't be of a large size. Again, best to validate this & proceed with caution. It is my self taught knowlege only.
Good Luck...