Caffeine is a portable free tool which can help to prevent your screensaver activating, or your PC going to sleep. Just launch the program and it simulates a keypress every 59 seconds, so your machine thinks you're still working at the keyboard, and won't shut down.
Caffeine adds an icon to the system tray where you can control how it works. Right-click and you're able to turn the program on for only an hour, for example. It will then keep your system awake for that time, convenient if you're not working at the keyboard but need it to remain active (you're giving a presentation, perhaps), before returning to normal.
The program provides several command line switches to control how it works. There are options to choose which keypress is simulated, for instance, and how often it's repeated. You can choose whether it's active on launch, or how long it will be active. You get to decide whether Caffeine works from your system tray, or a taskbar button, and there's even an option to prevent sleep, but allow the screensaver to start.
Conveniently, all this comes packaged in a single tiny executable (a mere 40KB) which, the author claims, runs on anything from Windows 98 upwards.