You're not talking about the Taskbar, you're talking about the QuickLaunch
bar, an optional part of the Taskbar. You can drag the icons to order them
the way you want them, but if the order gets lost, I don't think there's any
way to quickly get it back. You have to manually re-order them. If you
disable the QL bar and then re-enable it, the order is lost. The default,
when the manual order is lost, is to order them alphabetically.
I don't know what's causing your order to be lost unless it's that you
disabled/re-enabled the QL bar, but the order is stored in the Registry, so
perhaps yours is having some problem that causes that set of entries to be
lost and then recreated in default order.