Why is the 'Upload image' button greyed out?

I run a sort of membership site, and one of the options in a persons profile is to upload photo's. However one of my clients has the 'Upload image' button greyed out. Something I thought wasn't possible with standard buttons on a website.

The button calls a Javascript function that simply validates form data before doing a standard form submit. This only happens to them when attempting to upload images. There are other forms on the site that they have submitted successfully but these do not involve uploading.

I know it's not a site issue because I've had it tested in different operating systems and on different browsers.

I know it's not a specific browser issue because the client gets the same problem in both Firefox and Internet explorer.

This leads me to the conclusion that the problem is likely to be something to do with the operating system. Maybe security settings. They are running Windows Vista.

Has anybody come across this problem and if they did, how was it solved?