This is easy to deal with if you are removing the bullets from the whole list. It's more complex if you are just removing a bullet from one or two points in a list. If that's the case, let me know (here or by email) and I'll put the time into a LONG answer ;-)
In XP/2003 or in 2007, you can move tabs in the rulers...
XP/2003 it's in the View pulldown menu - select ruler. In 2007 there's a checkbox for it on the View tab. Then...
Click the hanging indent in the paragraph and you'll see a tab pointer show up in the ruler. Slide the pointer all the way to the left and the indent goes away. Depending on how the text box was created, doing this may fix every line, or you may have to do them individually. It's good to learn the ruler no matter what because that's where you control tabs and where the layers of bullets start, how much space between bullets and the beginning of a word, etc.
In 2007 you can also achieve this through the paragraph settings (I've never seen it in earlier versions). Select the paragraph, go to the Home tab, there will be a Paragraph box with a tiny arrow in the lower right corner - click on it to launch Paragraph dialog box. Set the indentation setting to None. This will remove the indent and align the type at the distance of the bullet indent (even without the bullet). If you want to align it flush to the left of the text box, also set the Before Text indent to Zero.
This will seem much simpler when you're doing it in PowerPoint vs. reading it here!