You probably use a screen saver on your PC, which kicks in every few minutes if you're not busy typing. Screen savers are fun and useful, but occasionally then can get in the way. If you're carefully reading something long and complex on your screen, for example, Windows isn't (yet!) smart enough to realise that, just because you're not typing, you're still using the computer. Alternatively, perhaps you're downloading a large file, or compiling a big program, and you want to keep an eye on progress without being distracted by an LCD aquarium.
If this sounds like your problem, then Keep Display On could be your solution. It's a small, free app for Windows XP and above. It doesn't need installation - just download it (it's less than 1 MB) and unzip it. Then, to use it, double-click its filename or icon. When it's running, the message as displayed in the screen shot below will be displayed. Which, as you can see, means that the screen saver won't kick in. It's as simple as that. When you're finished with that big download, or whatever else you were doing, close Keep Display On and your screensaver is back to normal operation.
One very useful feature of this program is that it doesn't need admin rights for you to run it, whereas changing the screensaver settings via the control panel often does. So it could be useful at work, as well as at home.
You can get KDO at http://www.dotnetniet.com/index_en.html.