Explorer can hang for minutes at a time, often taking other applications or your whole system with it. I have now seen this on at least a dozen installations. I have spent many an hour trying to track down what the root of the cause was. Here are some of the many possible causes compiled in a quick list so you don't have to suffer what I've suffered:

• *New*: I have received an email regarding ctfmon.exe causing Explorer to hang:
  "You might want to add that ctfmon causes hanging in Windows Explorer, particularly with IE7 (though when I downgraded to IE6, it still hung). Turning this off fixed things for me.
  
  More here: http://www.annoyances.org/exec/forum/winxp/t1167888467 [1]"
  
  In case you're wondering what in taranation ctfmon.exe is, it's the item Microsoft affectionately calls "Advanced Text Services" and is probably most familiar to you as the portion of the taskbar over on the right near the tooltray that has an icon of a microphone on it.
  
  I've never run into this is because I always disable the "Advanced Text Services" immediately on every Windows box I touch. Nobody I have ever known uses the feature, and it takes up CPU, RAM and taskbar real estate. This is just one more reason to kill it.
  
  To disable Advanced Text Services simply navigate to your Control Panel (in "classic view") and open the Regional and Language Options. From there navigate to Languages > Details > Advanced, and check Turn off advanced text services.
• Microsoft application settings are munged. Try logging on as a different user. Do the problems go away? If so, log on under your original account and move the folders named \Microsoft (or as much of it as you can, if files are locked and in use) out of your \Documents and Settings\Application Data and \Documents and Settings\Local Settings\Application Data folders somewhere safe (in case you need to restore them). I typically make a couple folders called C:\Temp\AD and C:\Temp\LS and drop them in there. I've had issues where Office applications lock up and also lock up Explorer when they open a file or attempt to open a file. Moving these settings instantly resolved the issue.
  
• Some Registry entries might point to a bad (old and no longer available) path. I often have this issue with cached network paths in my registry. I use RegSeeker [2] to search my registry for all instances of "\\" and review the results for network paths such as "\\computer.network.com\c$\program files\whatever\". I then delete all those paths (you should be sure you know what you're doing if you do this as some of your applications may require certain network paths). You should read this article if no other, as Mark takes the time to detail how to definitively track down these issues using freely available SysInternals [3] tools: The Case of the Intermittent (and Annoying) Explorer Hangs [4]
• There are any number of bad registry key scenarios that can cause this situation. I have had much success running the freely available RegSeeker [5] (from HoverDesk [6]) to quickly yank out the zillions of bad entries in a safe (recoverable) manner. The application has a feature "Clean Registry" which removes all the keys pointing to bad locations, but also saves all removed keys into a recoverable quarantine.
  
• You're using Windows 2000 or XP and Microsoft has not yet released a patch for this issue (yes, I have a special joy when I tell you this): Internet Explorer hangs when accessing files [7]
• You might have large .zip files in the root of your drive: Why XP Windows Explorer hangs when switching to D: drive? [8]
• The SSDP Discovery Service (for Universal Plug and Play Discovery) is frantically searching for devices and will continue to hang every instance of Explorer. Rebooting will not resolve this issue. Stop and disable the service under the Services console. I just experienced this one on my PC today and disabling the service instantly resolved the issu: Explorer exe hangs Computer problems again
  [9]
• It could be a buggy shell extension. I had this on a previous system and used some application which allowed me to enable/disable shell extensions. I'll post a link here if I come across it again.
• Some of the Windows system files may have been replaced. Perform an sfc /scannow from a Run... dialog or a command prompt (have your Windows install CD handy!) to have Windows scan all its system files and restore any replaced items.
• You should also check the signatures of all your Windows system files. Open a Run... dialog and type sygverif.
  
• Remember that Internet Explorer *is* Windows Explorer. They are the same application, and so their problems are often related. IE may have some mangled files, or you may infected with some sort of malware / spyware / adware. You may need to scan and remove some malware or even reinstall Internet Explorer: Internet Explorer hangs for 10 seconds [10]
• A google search on 'explorer hangs' [11]
• If you've exhausted all of the above, the final measure to take, short of completely re-installing, is the repair process (only available to XP/2003): Restart your computer with the Windows XP installation CD inserted, press [F8] when the licence agreement is shown and then press [R]. The setup routine will continue in the normal way, and any of your files that have become corrupt will be automatically replaced.