Windows 98/XP/ME/2000:
1. Minimise and/or close all windows so you can see your desktop background.
2. Right-click on the background picture.
3. From the menu that pops up, Left-click on the option that says "Properties".
4. Across the top of the page are a series of "tabs" and the last one says "Settings", go there by Left-clicking on the tab.
5. Near the bottom of the page you will see a slider with some numbers underneath.. Most likely, as ren suggested, you have it set to 800 x 600. Move the slider to the right and the numbers will increase. Set it to 1024 x 768 and Left-click the "Apply" button at the bottom.
6. Follow the on-screen prompts from here.
7. When you're done, Left-click the "Ok" button at the bottom.

Windows Vista:
1. Minimise and/or close all windows so you can see your desktop background.
2. Right-click on the background picture.
3. From the menu that pops up, Left-click on the option that says "Personalize".
4. At the bottom of the page that pops up, will will see the icon and words that say "Display Settings", Left-click on that.
5. Near the bottom of the page you will see a slider with some numbers underneath.. Most likely, as ren suggested, you have it set to 800 x 600. Move the slider to the right and the numbers will increase. Set it to 1024 x 768 and Left-click the "Apply" button at the bottom.
6. Follow the on-screen prompts from here.
7. When you're done, Left-click the "Ok" button at the bottom.
8. Close the "Personalize" window.

I don't own a Mac.. so don't know how to on a Mac.. but I think the only real difference is in the wording and method of bring up the menu.. For example, the slider that sets the screen resolution says "Screen Area" instead, I think..