Open paint. It is located under "Programs" and then "Accessories". After you have Paint open, press Ctrl+E. This will open the Attributes window (Attributes can be found under 'Image' too). Set width to 100, and height to 500. (Height can be any number, but 500 works well.) Make sure 'Pixels' and 'Colors' is checked and click OK.
Step 2: Draw the line
Your screen should now look like it does in picture 1. Now, select two colors you want to blend. Using the first color that you chose, use the line tool to draw a diagonal line connecting two corners, as in picture 2.
Step 3: Fill & Shrink
Using the bucket tool, fill the top half of the page with the line color and the bottom half with the other color you chose. Now, press Ctrl+W. This opens the Stretch and Skew window (this can also be found under 'Image'). Enter 1% into the horizontal stretch area. Click OK.
Step 4: Stretch it out again
Your screen should now look like picture 1. To stretch it out, again open the Stretch and Skew window. This time, enter 500% into the horizontal stretch area (500 is the max amount allowed). Click OK. Do this two or three more times, until your gradient is wide enough to be admired.
Step 5: Wavy Gradients
This step is a bonus trick I just learned. First, get your gradient to be relatively skinny. Then, use the corner dot to drag your page size so most of your screen is white. Make sure your secondary color white before you do this. Then, select your gradient using the rectangle select tool, and click transparency. Then, while holding the shift key, drag your gradient in any way you want, such as a wavy line. Lastly, use the fill bucket to fill in the white space around the edges. Voila!