There is this common (and very annoying) problem where an Explorer window keeps reappearing every time Windows logs in. Typically this will be "C:\Program Files\Common" or "C:\Program Files\Common Files". The problem often occurs after installing one program or another. If you check the startup items with msconfig, however, you will find no obvious reason for this. From the solution below I'm guessing that the reason is some bug in the handling of strings in Windows, where the string contains a pathname which includes spaces.
Anyway, here's the solution:
1. Run regedit
2. Find the registry entry
3. Go over the list of startup items. Most of these will be of the form:
(path)\(filename) <optional parameters>
Now, find all the entries where (path)\(filename) is *not* enclosed by quotation marks. Just add the missing marks and you're done! For example:
C:\Program Files\QuickTime\qttask.exe -atboottime
"C:\Program Files\QuickTime\qttask.exe" -atboottime
I hope this helps. If the info has been posted here before, apologies for the re-posing. Do not follow these steps if you are unfamiliar with regedit.