SiteUp will regularly monitor the websites you specify, and warn you if they go offline, for example because a server has crashed.
To set up the program, click the Add button and enter the web address of a site you'd like to check. Then enter a word or phrase that will appear on the page you've specified. SiteUp will check both that there's a page at the address you specify, and it contains your phrase, which means it should also warn you if the page is replaced by an error message.
Click OK, and set up the monitoring options to suit your needs. By default SiteUp will check your sites every 10 minutes, but you can cut this to 1 minute, if you like, which ensures you'll be alerted of any problems at the earlier possible opportunity.
Be aware that every check does require some use of your internet bandwidth, however, as SiteUp needs to download the full text of your specified URL. This probably won't be too much, but if it's an issue then you can always increase the checking interval again later.
Once you've done, click "Close and Start Monitoring" and SiteUp will minimise itself to your system tray. Hover your mouse cursor over its icon for a quick status check, or right-click the icon for configuration options or to close the program down. Don't worry too much about leaving it running, though - when minimised SiteUp consumes only a little more than 3MB of RAM (private working set).