If you are looking to get a smaller desktop systems and not spend a lot of money, it is hard to top the $329 price of the Acer Aspire X603. This slim desktop system that could actually be smaller if Acer had opted to use a different case than their previous models. The Pentium J2900 is an unusual processor but it does provide four cores for some decent performance. The big thing holding back the Acer system though is expansion. Even with the internal space, the motherboard lacks most connectors which prevents things like graphics cards from being istalled. As a result, you need to be certain the machine will meet your needs. If you need something with a bit more capability and can spend more, the Dell Inspiron 3000 Small can be found for around $400.
I have loved the Apple Mac Mini for years but Apple has just let the system slide for the past two years. Because of this, I think the Lenovo Q190 is a better option for those looking at a mini PC. It may not have as high of performance but most people are looking for a compact system that will be used for browsing the web, streaming media and doing productivity applications and the Core i3-3217U dual core processor handles this just fine. The system is actually smaller than the Mac Mini and offers a near silent operation. It even comes with a keyboard and mouse. Pricing is around $549. Of course, the Mac Mini is always a solid alternative for just $50 more.
The slim tower desktop design seems to be less and less popular with mainstream systems and instead is being used for gaming systems. If you are looking to not spend too much on a system but still want some good performance, the Alienware X51 still offers a great value. Pricing starts at $699 but the $899 version is the one I recommend because it features a Core i5 quad core desktop processor along with a dedicated GeForce GTX 745 graphics. This makes it less suited as a dedicated gaming system but still provides it with a well balanced system. The best part is that one can also get it with Windows 7 rather than Windows 8. If you don't necessarily need the graphics card and want to spend less, than the Dell Inspiron 3000 Small is a nice alternative.
There are a number of excellent choices out there for those that are looking for a compact desktop system that offers the full performance of a desktop for PC gaming in a home theater environment. My personal choice for this type of system is the Digital Storm Bolt II. This is an updated version of a compact tower system that offers some outstanding performance thanks to a Core i5 quad core processor, GeForce GTX 750 graphics card and a solid state drive. Many companies still do not ship SSD's standard with there systems which is what sets Digital Storm apart. The price tag is quite high at roughly $1700 and the front audio and USB ports are in a slightly awkward place. A slightly more affordable option that comes in a variety of different sizes is the Origin PC Chronos that can still game at 1920x1080 resolution of most HDTVs but for hundreds less.
There are plenty of Smart TVs out there on the market that can stream from select services but one of the features they lack is the ability to browse the web. This is where a tiny device like the ASUS Chromebox can really expand a home theater system to be more than just a SmartTV. The ChromeOS essentially lets you do anything on the web that could be done on a computer plus a bit more. With a starting price of under $200, they are extremely affordable but you need to add a wireless keyboard and mouse to full use it. Also, it only has 16GB of internal storage so expect to store a lot in cloud services like Google Drive. Just be warned this is not a full blown PC experience. It is similar to the Chromebook but this looks to be changing a bit more in coming months when Google eventually releases some Android apps for the Chrome OS.