Have you ever had trouble sending a particular file attachment through e-mail, and wondered why? The file is probably too large. Most mail servers (both incoming and outgoing) restrict attachment sizes to a maximum of 10MB, and sometimes even less than that. Gmail and Yahoo are among the exceptions, but even they limit things to 25MB and 20MB, respectively. If you're trying to send someone a video file, a large batch of photos, or anything else that exceeds the limit, the e-mail won't go through. (Think of it as trying to stuff an oversize package into a small mailbox.)
Fortunately, lots of free services let you work around these restrictions. For example, YouSendIt lets you send files as large as 100MB via a Web interface, a desktop utility, or even an Outlook plug-in. Another great option is SugarSync, which offers 5GB of free online storage and makes sharing big files easy. I also like WeTransfer, a dead-simple--and dare I say, sexy--browser-based tool for sharing files in sizes up to 2GB.
(For more solutions to perplexing PC problems, see "The 21 Greatest PC Mysteries--Solved!")