Cannot send attachments by email

Ever since I upgraded from Windows 98 to Windows 2000, I have been unable to send an attached file of any size in an email message. This occurs when sending from both Outlook Express and web-based mail (e.g., Hotmail), both in Internet Explorer and Netscape. By “any size,” I mean something larger than 50KB or so. Has anyone incurred this problem or seen a solution to this?