This is an Explorer problem, although I don't suffer myself with it but it is related to Ram.You can install FreeRam XP which reclaims ram from apps that are closed and this pretty much cures the problem.
Another thing you can do although this is not a cure but it might help a little, instead of rebooting you can open up the Task Manager by right clicking on the taskbar, then in the Task Manager selecting the explorer.exe and End Process.Then in the Applications tab of the Task Manager hit New Task and type explorer.exe to restart the desktop.