JCalc

JCalc is a simple calculator developed in Java, which can be used by those who have trouble getting the Windows Calculator up and running to perform some computations quickly.
Before proceeding any further, you should know that it doesn't offer support for advanced functions, such as trigonometrical operators, so you have to resort to a different program if you're interested in these aspects.

Needs Java to run

The full package consists of just one executable .jar file that you can save anywhere on the disk or copy to an external storage unit to seamlessly launch JCalc on any machine running Java. It doesn't update the Windows registry with new settings or create any new files on the disk. To uninstall it, you only have to delete this item.

Basic math calculator with little to offer

As far as the interface is concerned, the utility has a single panel that shows all the calculator's buttons, giving you an overview of all operators available. Besides addition, subtraction, multiplication and division, you can find out the square root of a number. Decimals are supported while the current task can be cleared with one click. Calculations are executed instantly.

Doesn't support keys or richer features

On the other hand, an inconvenient aspect of this application is that it doesn't support keys. Therefore, it cannot execute commands when attempting to type keys or operators.
Apart from this fact, JCalc doesn't bring anything new to the table. Instead, it's a rather rudimentary calculator that has little to offer. For instance, it doesn't show complex expressions but clears the current number as soon as a new one is entered, making tasks difficult to keep track of.
On the other hand, it didn't hog system resources in our tests, running on low CPU and RAM. It's free for non-commercial use.