UPDATE: Many Windows 7 users are facing this problem. This problem occurs when you enable Quick Launch toolbar and it automatically disappears after reboot.
Some of Windows 7 and Server 2008 R2 users are facing a weird issue now-a-days. When they log off or restart their Windows 7 computers, Taskbar resets to default settings and forgets all their customized settings.
According to Microsoft KB979155 article, it happens because of a race condition between Windows Explorer and Taskhost.exe.
This issue only occurs when "Automatically Log On" feature is enabled. Some people enable this feature to automatic log on to their systems without the need of entering password at login time.
If you are also facing this issue, you can ask Microsoft Support for a hotfix. Its not available for all users, so you'll need to ask them for it.
Or you can follow these simple steps to disable "Automatically Log On" feature which will also fix this issue:
Method 1: Using "User Accounts" Applet
1. Type control userpasswords2 or netplwiz in RUN or Start Menu Searchbox and press Enter. It'll open "User Accounts" window.
2. Now enable "Users must enter a user name and password to use this computer." option.
3. Click on Apply button and it'll fix your problem.
Method 2: Using "Registry Editor"
1. Type regedit in RUN or Start Menu Searchbox and press Enter. It'll open "Registry Editor".
2. Now go to following key:
3. In right-side pane, change value of AutoAdminLogon to 0 or simply delete it.
4. That's it. Restart your system and it'll fix the problem.
PS: If you are not familiar with Registry editing and want a ready-made script to do the task automatically, download following zip file, extract it and run extracted REG file. It'll fix the problem for you: