Vista - Can't open email attachments

Vista x64 Windows Email won't always open attachments. During the same session, will open for awhile and will not open. No error message, just won't open. I've changesd the attachment option under security, checked "open with" settings and even changed from McAfee to Norton. I can save the attachment and then open directly from the file. I've seen several problems listed about open attachments but none that was intermittant. Any ideas?
Thanx