A quick search found this result...
It was suggested that the 'disappearing' quick launch toolbar is a result of malware (Spyware / Adware) which has overwritten a registry key, causing Quick Launch not to appear after a reboot. Resolving this issue involves resetting a value in the System Registry. I have not tested this on my own machine, but a follow-up reply in the techguy forum suggests that the fix works. To quote:
" In Registry Editor (Start > Run... > regedit), navigate to: HKEY_LOCAL_MACHINE \ Software \ Microsoft \ Windows NT \ CurrentVersion \ Winlogon. Look for 'Name: Userinit'. In that, you will see 'Data: %system32%\wsaupdater.exe,'. Note that %system32% represents the path to the System32 folder. For example, if the path is C:\Windows\System32, then the data would be: 'C:\Windows\System32\wsaupdater.exe,'
Instead of wsaupdater.exe, the data should contain userinit.exe, (including the comma at the end). Using the example above, the data should now read: C:\Windows\System32\userinit.exe. Make the change accordingly; close the Registry Editor and reboot your computer. "
Side note: always backup your registry