Hotmail Attachments

I recently upgraded to a new computer with Windows XP Pro and McAfee Security Center. Since switching to the new machine, whenever I try to download and open a file (say a .doc or .xls file)sent as an attachment via my Hotmail account, it takes me back to a Hotmail sign-in window. This has never happened before. I still have plenty of memory on my Hotmail account, so I don’t think that’s the problem. I have been unable to get a response from Microsoft, so far. Any suggestions?