Very strange client/browser issue

One of our clients has logged a very strange issue with us- We launched a preview for their website, but when it's viewed on their main PC, peculiar things start to happen...

At first, the stylesheet wasn't being found, and so accessing any page resulted in one void of all styles. We sent them a direct link to the stylesheet, which was viewable from all our computers in the office - but gave a "File Not Found" error on their side. I then deleted the file, and replaced it with a new blank file, which he could then access. Copy-pasted screen.css contents into this file, and he could then view it fine, and stylesheets magically worked on the site again.

Now, he can view styles, but not the referenc开发者_StackOverflow社区ed header images. The strange thing is that this problem doesn't exist on any other PC we've tested, or on any other site on the problem computer, but obviously we'd like our client's site to work for them. The strange thing is, they can view other sites of ours, hosted on the same server, built on top of the same CMS (and so most of the files are the same) without problem - but are getting 404s for files that most definitely do exist.

• Stylesheets are not turned off, nor is anything specifically deactivated on their browser (as other sites are fine)
• Reloading with CTRL+F5 doesn't help
• The client is using the latest version of firefox

Any ideas here on what to try / how to narrow the problem down?

Edit:

Client re-installed browser for a second time, and the problem seems to have rectified itself. Still no idea what was causing the problem, but it is, at least, gone for now. Thanks to everyone for their suggestions, I don't think I can really accept an answer, but I've +1'ed all of you for the help.

If they are getting 404 responses these must be being sent from somewhere! Can you use Fiddler to monitor the traffic at the client end to check the requests are going to the right place (apparently) and that there is nothing obviously awry with the request.

And look in the server logs to see what requests are coming in (are they being altered by a proxy?) and why it is sending a 404 (and indeed if you can match the requests at all and there isn't some odd DNS issue at the client end).

Edit If you aren't actually at the client site there is a cut down version of fiddler you can get the client to use to make a capture that may be easier for them.

If they are using any Firefox plugins, try disabling them (Tools -> Add-ons).