I would try these steps.
1] Use a temp file cleaner like CCleaner from http://www.ccleaner.com to clear out all temp and TIF files, (Temporary Internet Files).
2] Look at the Hosts file on that computer. Hosts, (no file extension), is found in the C:\Windows\System32\Drivers\Etc folder and is a file that all browsers check first before searching for a web site on the internet. Typically a hosts file will redirect a browser to the home computer itself, so the browser shouldn't display a 404 page, just a blank page, but it is worth a look.
3] Are there any anti-spyware utilities running that would block certain web pages?
4] Do these sites work on other computers in the office?
Others here may have better suggestions.
I've been having an issue with one of our machines in the office. The problem arrose fairly recently (about a week and a half of so,) everytime the user tries to access Yahoo! Mail Internet Eplorer gives a 404 error. In general surfing there appears to be no problems at all, everything works as expected. I figured possible that the user had blocked something with their personal firewall, however this does not appear to be the case. I tried to access a few other mail sites as well. Hotmail worked just fine, but Gmail gave the same odd 404 error. I then procede to try to install a 3rd party web browser, thinking possibly it was a problem with IE. I installed Firefox (not sure which version) and still on Gmail, and Yahoo Mail I still get the 404 error. Any Ideas?
The machine is a Dell(not sure of the model i can find out if necessary) running XP Home with Internet Explorer 7
If there is any other information that may be needed I can certainly try to get it in as timely a manner as possible. I'm not really in IT here. We are a small enough company I just happen to be one of the more knowledgeable ones.