Watching a movie at home is often a spontaneous activity. Maybe you're entertaining houseguests, got the kids to bed early or realize that there's nothing good on TV and you decide, "hey, let's watch a movie." You can flip through your collection of DVDs that you've seen a hundred times, or you can rent something you haven't seen.

Unfortunately, renting a DVD is not as spontaneous as the decision to watch a movie in the first place. Nor is it very convenient in today's I-want-it-right-now world. You've got to trek out to the movie rental store (if you can still find one) or grocery store kiosk, not even knowing if they have the title you want in stock. Or there’s always the rent-by-mail deal, which is more convenient, but it has its downsides too. What if you're not in the mood for the title(s) that you selected days or weeks before when you were putting together your "queue"? Wouldn't it be perfect if you could rent on demand without having to leave the house or wait for a DVD in the mail? Now you can!

Enter online movie rentals.

Until now, the low quality and limited playback options of movie downloads—and the trouble of getting movies off a PC or portable player and onto the big screen—have made them impractical for home theater use. But a new wave of Internet-ready set-top boxes from companies like Apple, TiVo, Netflix and Vudu give you the ability to instantly download and playback movies that look and sound as good as the shiny discs that currently rule the home theater roost. Best of all, they let you browse thousands of movie and TV titles, select one for rental or purchase and watch it right on your family room TV in seconds without a PC. It couldn't be easier or more convenient.

These new online services will transform the way you think about renting and buying movies. Here's what they offer:

• Browse thousands of titles, watch previews and purchase or rent on your TV screen.
• Rentals cost between $1 and $6, except Netflix, which is an all-you-can-watch subscription.
• Content selection varies, but generally includes thousands of movies and TV shows, and all services offer titles in high-definition.
• Most services let you begin watching a movie within seconds of selecting it.  This of course, will vary based on broadband speed, Net traffic, and so forth, but it is, in most cases, near instantaneous.
• Downloads are stored on internal hard drives giving you instant access and easy search capabilities for the titles you purchase.
• For most rentals you have 30 days to begin watching (14 days for Microsoft), and 24-hours to finish a title once you’ve begun (these rules are set by the Hollywood studios, not the service providers). Netflix has no time restrictions, similar to its mail-rental plans.
• No need to return or delete the movie, it erases itself!

Online movie rental downloads aren't quite ready to replace your DVD player yet, but for many consumers' busy lifestyles, these services are a perfect addition to the family room home theater. You still have your DVD player for watching the discs you already own or that a friend might bring over for movie night, but you also have the option of a new service which gives you spontaneous and instantly gratifying access to a decent selection of titles.

Online Movie Rental Set-Top Boxes At-A-Glance: Apple offers movie rentals on its AppleTV set-top player Microsoft offers movie rentals through the online marketplace on its hugely popular Xbox360 gaming console. TiVo has partnered with Amazon to offer online rentals through its Series 2 and Series 3 digital video recorders. Vudu offers a sleek-looking set-top box that gives you exclusive access to its online rentals and store. Netflix now offers instant viewing of a select number of its 100,000 titles using its Netflix Player By Roku™ set-top player.

There are some downsides to downloading movies, most of which are a function of the movie studios' Kung-Fu grip on their intellectual property and the digital rights management (in lay terms, copy protection) that limits how it can be used. What this means for you is that movies are, in most cases, not transferable to other players or devices. Apple has secured rights to let you transfer rentals to an iPod or iPhone for portable viewing; however, no other services currently let you view a rental or purchase on anything other than the set-top box itself. The 24-hour viewing window (N/A for Netflix) could be an issue for some people, but certainly not a deal-breaker for most. Also, hardware choice dictates service. Unlike DVD players--which must play all commercially produced discs bearing the DVD logo--these boxes are proprietary. Whichever set-top box you purchase, you only have access to that company's service. Some services have larger rental selections than others, and pricing varies slightly.  If you enjoy the "extras" often found on DVDs, you may find renting "just the movie" disappointing. Finally, copy protection prevents you from archiving the movies you purchase, so if your device fails, your movies are gone.