If it is necessary to determine the on-time of the computer that is running Windows Vista or Windows 7, then it can be done by using a system component called "task Manager". To run it click a free space on the taskbar, right-click and select in the popup context menu, which is called "task Manager". And you can use the combination CTRL + ALT + Delete.
Go to the Performance tab, and, among other information, look in the section "System" the line "working Time". Subtracting from the current time period specified in this row, you can determine the time the computer is turned on.
Use the utility systeminfo, if you want a method which applies to Windows XP too. This tool works in command line, so it should start with opening a terminal command line. Open the main menu on the "start" button and select "Run" to open the window, run programs. It can do the same by pressing the key combination WIN + R. In the entry field, type cmd, press Enter and the system will give you the opportunity to use a DOS emulator commands.
Type into the command prompt, systeminfo. You can highlight and copy (CTRL + C) the name of the utility here, and then click the right mouse button a black terminal screen and in the popup context menu select "Paste". Then press Enter and the tool will collect information about your system. This process will take a few seconds, and then the terminal screen will display a long table with various data.
Scroll to the top of the table and find the line "system boot Time" - and it will be the required switch-on time. But this line is only in Windows Vista and Windows 7, and Windows XP instead, there is the inscription "the system", so you will need to take the specified time from the current clock.