window check exist

I have a prog i use to send keys to a specific window name..
but if i enter a invalid name e.g (Untitled - Noetpad) it will fall over with argument exception( of course.. )
is there somthing that i can use to check the window exists before 

appactive(winname)


pseudo
if winname does_not_exist then
msgbox("blah does not exist")
else
appactive(winname)
SendKeys.Send(keystxt.Text)
end if


thanks

On Error Resume Next