i had the same problem with the disappearing taskbar icons but i followed this advice

Open avast antivírus and go to configuration. There's a menu inside avast antivírus. Above general, there's a option that show Avast icon. If checked, un check it. May be you need re-start. When re-starts, re-do this and check it.
and after several reboots on my XP SP2 system it has not reoccurred.