Is there a way to turn an HD TV into a smaller computer monitor?

This is a general answer that applies to using TVs as computer monitors in general.

• If the TV has a digital input (HDMI, DisplayPort or DVI) use that. Never use the VGA input on a TV unless there is no other option available.
• Most TVs have "image quality improvement" settings somewhere in their menu. You want these to be OFF when using a TV as monitor because the computer already provides a "best quality" signal. Any further processing by the TV will only degrade quality instead of improving it.
  (This is often the most noticeable reason for a "noisy" looking image.)
• As @pbhj already mentioned in the comments you don't want the TV to do any sort of scaling/clipping on the computer image as that introduces noise/blurriness too. So you will want to disable over-scan and may have to experiment with display ratio (4:3, 16:9, 16:10) and various settings for letterbox/cinema display.
• Some TVs have a pre-defined preset mode for use as monitor (often called PC-mode). If that is available, use it. It should make the various adjustments I mentioned in the 2 previous points above.
• If you have your Operating System set to scale its display to more than 100% (125% and 150% are common settings in Windows), set it back to 100%. This usually gives a more crisp picture, which is especially noticeable on a lower DPI screen like a TV.
• If your computer uses ClearType (Windows does) to improve text quality you will want to disable (or at least re-calibrate) it, because the TV will require different settings than the original monitor did.
  This is not a TV specific thing: ClearType needs to be adjusted for each monitor and display scale-factor (see previous point) individually to get the best results. Many people don't realize this and keep working with sub-optimal settings. On high DPI screens sub-optimal ClearType settings are not that noticeable, but it is a lot more obvious (blurriness of text) on a low DPI screen like a TV.

Last but not least:
TVs have larger pixels (lower DPI) than a proper monitor.
This will give many people the feel that there is something wrong with the picture even if they can't quite tell what exactly bothers them about it.
In general this is due to the fact they sit too close to the low DPI screen and can see the individual pixels. The only remedy for that is placing the screen farther away.
E.g if you go from a 24" monitor to a 48" TV, at the same resolution and place the TV in the same spot as you had your monitor, the TV will have pixels that are appear to be 2x as big.
In order to get the same sort of viewing experience that you had with the monitor you will have to place the TV at 2x the distance from your eye. (That can be problematic if your desk isn't deep enough.)
A little closer will probably still work. For most people somewhere between 1.5x and 2x further way is comfortable. It is also something you get used to after a few hours/days, so you can maybe bring the TV in closer after a while.