Executes for me (WinXP Professional Edition, mIRC 6.03).
Don't want to be patronising or anything, but make sure you only make the timer execute 1 minute before the time in your system tray (for example, if it says 13:21 then do /timer1 13:20 blah blah). Make sure you got v6.03 too...
Otherwise, weird if it's affected by Windows OS....I'm going to complain to Bill!
Regards,