"There was a problem checking available updates ..." for Drupal, modules or themes

I have used both IE and Mozilla FireFox in Windows XP. Both of them got the same problem:
Two lines of "There was a problem checking available updates for Drupal ..." showed up at the top of Modules page. When I went to Administrator/Update page, it showed, "All of your projects are up to date." However, when I clicked on "check manually" button, I got "Failed to get available update data for 9 projects."
How to fix this "available update data" problem? Please help.