In other words, make it short but informative. If you're writing to Hassle-Free PC, for instance, give a quick description of your problem. A great example: "Computer not recognizing DVD/CD-ROM drives."
Remember, the subject line of your e-mail is the first thing the recipient sees. It should be a tiny summary of what's in the body of the e-mail. Anything else and your message might get overlooked, passed over, or just plain ignored.