Location for Google search and Firefox wrong

On a workstation I administer when ever the user opens Firefox or use Google to search for something both think I am in the Netherlands rather than the US. I have checked the location and language settings in both Firefox and Windows 7 and both are set correctly for US and English. This oddity started at the same time the user picked up "My Security Shield" malware which I was able to remove. Currently both Antivirus and malware scans show the system as clean. Would like to avoid a reinstalling the OS if I can so any help would be appreciated.

First thing to check: is Google using some cached user preferences which have changed?

If Google isn't setting the language based on a user preference, there are other ways it could be guessing the preferred language.

Web browsers send an Accept-Language header string as part of the request, and if 'nl' is listed first, like this:

Accept-Language:    nl,en-us;q=0.7,en;q=0.3

then Google will display a Dutch page.

I know you said you checked this in your question, but are you certain that the Firefox language preferences (Tools -> Options -> Content -> Languages) don't list Dutch [nl] among the options?

You can test what headers the browser is sending by visiting http://www.xhaus.com/headers

The other possibility is that the PC has been assigned (by the ISP) an IP address which appears to be in the Netherlands, or that web requests are being routed through some external proxy (hosted outside the US) which you're not aware of.

Try visiting http://www.geoiptool.com/ and see where it thinks you are.