All my browsers suddenly don't support Javascript files anymore?

This is way out there but have you tried connecting directly into your modem and removing your router as a possible source of problems?

I stumbled on this recently when I was randomly unable to install windows updates from their website. It didn't even occur to me to rule out the router originally as web pages were loading fine.

It just so happened that I was planning on replacing the router anyway. After the swap I noticed I wasn't having the problem any more so I began suspecting the old router as a problem. Sure enough I plugged it back in and after awhile I would experience strange (but non reproducible) errors.

Glad to see you figured it out!

When troubleshooting an issue like this you need to check everything from end-to-end:

1. Make a back-up if you haven't done already, you might lose personal data otherwise...

2. Get troubleshooting tools that show you what happen under the hood:

   • Fiddler2, a web debugging proxy, allowing you to see what happens to the requests.

   • Wireshark, to go to level of packets to figure out whether there is something odd there.

3. Do a complete manual reset of Internet Explorer including any personal data or settings.

4. Start Fiddler2.

   Identify what happens with the Javascript request, surf cache-less to http://superuser.com.

   

   In my case, I got return code 200, which according to RFC 2616 - HTTP/1.1 stands for OK.

   Now, in your case I suspect two things might be seen here:

   • The request might not happen, because:

     • The source code of the web page has been modified not to download the .js file.

     • The browser is not allowed to request the .js file, by means of a local firewall.

   • The request did happen and get a 200 OK response, but the body is invalid.

     You can see the body by clicking the .js event and then clicking the body tab in the bottom right section, here you should see Javascript like code. If not, something's tampering with it...

   • You get a return code indicating the problem.

The exact thing that happens here would give us a direction to search for...

If you have suspect packet level problems, fire up Wireshark if you know what you're doing.

1. If you have it, use a LiveCD or another computer/installation to verify if it's happening there.

2. Confirm that other users of the same ISP in your neighbor as well as people in your appartment have no problem, it's important that you need to know that it's-just-me-and-nobody-else before you continue doing work on your end of the connection.

3. Ask your ISP to reconfigure your modem, just to ensure the problem is not there.

4. If you host your a router with a firmware like Tomato or DD-WRT, check it's configuration.

5. Disable any firewall (router, software, windows firewall) to pinpoint whether a firewall is the issue.

6. Disable any virus scanner (also spyware, ad-ware, defender) to pinpoint whether they are the issue.

7. Make sure your computer is up-to-date (Windows, Drivers, ...), also check for third-party code:

   • Automatic Runs, look for non-Microsoft things that are starting, especially network related ones.

     

   • Process Explorer, look for non-Microsoft things that are running, use the Company Name field.

     If you know what you are doing, you might also check out the DLLs.

   • Go to the settings of your network card, look at what items are being used.

     

   • In the Device Manager, check the lower/upper filters in the details tab of your card's properties.

8. Reset your network settings.

   Execute the following commands, put them in a .bat file and run it as administrator:

   netsh winsock reset all
   netsh int 6to4 reset all
   netsh int ipv4 reset all
   netsh int ipv6 reset all
   netsh int httpstunnel reset all
   netsh int isatap reset all
   netsh int portproxy reset all
   netsh int tcp reset all
   netsh int teredo reset all
   ipconfig /flushdns
   ipconfig /release
   ipconfig /release6
   ipconfig /renew
   ipconfig /renew6
   ipconfig /registerdns
   

9. Check for tampering to your network settings.

   Similarly, execute

   netsh dump > "%HOMEPATH%\My Documents\netsh dump.txt"
   ipconfig /allcompartments /all > "%HOMEPATH%\My Documents\ipconfig dump.txt"
   route print > "%HOMEPATH%\My Documents\route dump.txt"
   

   and inspect the dump files in My Documents.

10. Try creating a new user profile, in some very odd occasions it actually works.

11. Abandon ship! It seems you can't use it where you live. Don't worry, Google is working on Dart...

But, lessons learned...

Proper back-ups are invaluable and the easiest fix to this kind of problem!

Well, if your ISP or Apartment is not trying to fool you in a nerdy way.