If Vista the keyboard and mouse actively sending information in can be interpreted as activity, thereby resetting the timer and preventing the screensaver from activating. The test is to disable the keyboard and mouse, such as removing the batteries, or replacing them with wired USB or PS/2 counterparts. Also, check the keyboard and mouse's manufacturer's website for updates to the software which may address this issue. It could be other things, such as your internet connection, but investigate the wireless peripherals first.

John