It really depends on what you mean by focus. If, following your example, your Email client appliation that was sitting minimised on the taskbar and then suddenly jumps up to full screen and checks for mail then you need to try and run that scheduledtask in "hidden" mode. That is where the mail checker works solely in the background and is hidden from the user. If you use Outlook or outlook Express on a LAN there is no popup or hanging, just a message somewhere to say you have mail.
If you use a modem and dial out to send and recieve mail then this may cause a delay as the modem will hoggs the PCI bus and causes the rest of your machine to hang momentarily. This is due to the PCI latency. You can alter this value in the BIOS but you would end up trading off bandwidth to availability. Some motherboards (ASUS) allow the latency to be setup per slot whereas most only allow per bus. I wouldn't do it unless your happy you can recover your existing settings and you understand what your doing.
If I've missed the mark with this give more details.