Its simple. Position the cursor to the place in the timeline that contains the frame that you want. Then click on the Preview Window. In the Preview Window there is a button that looks like two sheets of paper being copied. Click on this button to copy the frame into the Windows Clipboard. You can hover the mouse over the buttons to display ballon help if you can't find the Copy button. Then, in your paint program, use the program's paste function to insert the clipboard's contents.
You may also want to right click the preview and make sure that "Display at Project Size" is selected (If you want to save as 720 x 480). You may also click on the disk icon and save directly as either a .jpg or .png file.