Assuming you're using Internet Explorer:
1. Carefully compare the security settings for the 'zone' these sites are in.
2. Go to www.mozilla.org, download Firefox (another browser) and try if it works there.
Hope this helps.
I've been experiencing this problem for the past few months now. I cannot attach any files to any of my emails when using any online web service such as hotmail, gmail or yahoo. When I go to click on attach nothing happens and after repeatly clicking on attach still same thing. Also I noticed when sending data such over the net such as when logging in it takes longer then normal. When Trying to post or edit posts it also takes very long. Sometimes I cannot even edit my posts at all.
I'm using windows xp pro with service pack too. The problem seems to exists only on my desktop as I tried to attach files on my laptop and everything is fine.