Wiggling the mouse

The "correct" way to do this is to respond to the WM_SYSCOMMAND message. In C# this looks something like this:

protected override void WndProc(ref Message m)
{
    // Abort screensaver and monitor power-down
    const int WM_SYSCOMMAND = 0x0112;
    const int SC_MONITOR_POWER = 0xF170;
    const int SC_SCREENSAVE = 0xF140;
    int WParam = (m.WParam.ToInt32() & 0xFFF0);

    if (m.Msg == WM_SYSCOMMAND &&
        (WParam == SC_MONITOR_POWER || WParam == SC_SCREENSAVE)) return;

    base.WndProc(ref m);
}

According to MSDN, if the screensaver password is enabled by policy on Vista or above, this won't work. Presumably programmatically moving the mouse is also ignored, though I have not tested this.

for C# 3.5

without notifyicon therefore you will need to terminate this application in task manager manually

using System;
using System.Drawing;
using System.Windows.Forms;

static class Program
{
    static void Main()
    {
        Timer timer = new Timer();
        // timer.Interval = 4 minutes
        timer.Interval = (int)(TimeSpan.TicksPerMinute * 4 / TimeSpan.TicksPerMillisecond);
        timer.Tick += (sender, args) => { Cursor.Position = new Point(Cursor.Position.X + 1, Cursor.Position.Y + 1); };
        timer.Start();
        Application.Run();
    }
}

When I work from home, I do this by tying the mouse cord to a desktop fan which oscillates left to right. It keeps the mouse moving and keeps the workstation from going to sleep.