This api will do what you want:
http://www.mentalis.org/apilist/keyb_event.shtml
Example(from the site, but modified to work in .net:
Const VK_H = 72
Const VK_E = 69
Const VK_L = 76
Const VK_O = 79
Const KEYEVENTF_EXTENDEDKEY = &H1
Const KEYEVENTF_KEYUP = &H2
Private Declare Sub keybd_event Lib "user32.dll" (ByVal bVk As Byte, ByVal bScan As Byte, ByVal dwFlags As Integer, ByVal dwExtraInfo As Integer)
Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load
Me.Show()
Do While Me.Created
keybd_event(VK_H, 0, 0, 0) ' press H
keybd_event(VK_H, 0, KEYEVENTF_KEYUP, 0) ' release H
keybd_event(VK_E, 0, 0, 0) ' press E
keybd_event(VK_E, 0, KEYEVENTF_KEYUP, 0) ' release E
keybd_event(VK_L, 0, 0, 0) ' press L
keybd_event(VK_L, 0, KEYEVENTF_KEYUP, 0) ' release L
keybd_event(VK_L, 0, 0, 0) ' press L
keybd_event(VK_L, 0, KEYEVENTF_KEYUP, 0) ' release L
keybd_event(VK_O, 0, 0, 0) ' press O
keybd_event(VK_O, 0, KEYEVENTF_KEYUP, 0) ' release O
Application.DoEvents()
Loop
End Sub