There are various special cases, but in general:
You include an article in English whenever you have a singular
common noun (which usually refers to a class of entities (city, planet, person, corporation)).
You omit an article with a proper noun (which usually refers to a
unique entity, such as London, Jupiter, Sarah, or Microsoft).
Articles are optional with plurals -- not in the sense that you can include them or not on a whim, but that they change the meaning.
You should also not use an article when you use a possessive, like "my", or the adjective "one". (I'm sure I've seen other adjectives that take the place of an article, but I can't think of any right now.)
Maybe that's not entirely clear, so here are some examples:
You can start the application. "Application" is singular, so an article is required.
You can start applications. "Applications" is plural, so you don't need an article.
You can start Microsoft Word. "Microsoft Word" is a proper noun, the name of one particular application, so no article is used.
You can start one application. "One" takes the place of an article.
You can start his application. "His" is a possessive that makes an article unneeded.
If you use "the" before a plural, it indicates that you are talking about some particular set. Like if you said, "You can start applications", that would mean any applications, but if you said, "You can start the applications", that would indicate that you are talking about some specific set of applications, not any applications in the world.
In your example, I would have written "You can log in to THE desktop". "Desktop" is common noun and so requires an article. Or perhaps in this case the writer is referring to some specific product called "Desktop" and so he is thinking of it as a proper noun. If that's the case, it should be capitalized.