I see the same. As W7 doesn't require a reboot "by the clock" I also see multiple instances running. I just kill them. This looks like a small bug to me. Same for Internet Exploiter btw.
Never experienced this situatuin myself. Try this to see if it helps: Run "CMD" as admin, type "sfc scannow"; hit enter. Reboot and see if problem persists.
I can confirm this problem while using Firefox 3.6.8. "Open Containing Folder"(right click on a file) in the Downloads window(ctrl j) will open a new instance of Explorer each time it is used. The problem lies with Explorer stays in memory when closing the Explorer window. So after examining 20 downloads I will have 21 copies of Explorer in memory even with all Explorer windows closed. Some were blaming uTorrent for this same thing. But it happens with Firefox too. I can't blame either program for Explorer not being able to close itself properly. Edit: I just pinned Explorer to the taskbar. I get a new instance of Explorer in memory each time I click on the icon. And all the instances of Explorer do not free up memory when they are all closed. I did the same thing on my x64 desktop and only one instance of Explorer.exe was present no matter what I did. Something got broke.
7 x64 here. I only have one explorer.exe unless I open another window, and it ends when I close it. I haven't tried the Utorrent thing, I rarely open the containing folder from within utorrent. Edit: Even when I open another window I still only have 1 instance.
locate the original File C:\Windows\explorer.exe and create another short cut from that. It seems to have worked with my version of win7 also check properties on the short cut make sure it just says C:\Windows\explorer.exe.... I used to add /N,/E,C:\ on the end to take me straight to the C: drive but since I've removed it, no more multi instances of explorer.exe have occured.