Scite

Scite is a Scintilla-based text processing tool that supports syntax highlighting. It does not require installation.
Since this is a portable app, you can save it anywhere on the hard disk and click it to run. It is also possible to move Scite to a USB flash disk or similar storage unit, in order to run it on any machine with minimum effort.
What's more, the Windows Registry does not receive new entries, and leftover files are not kept on the HDD after removing the utility.
The interface is comfortable to work with, as it is formed from a regular window with a minimalistic layout. You can copy text as RTF, set bookmarks, complete a word, insert or expand abbreviations, block comments, as well as convert a selection to uppercase or lowercase.
In addition, you can use a find-and-replace or incremental function, enable whitespace mode, set parameters, switch to full screen mode, wrap text, activate read-only mode, change indentation settings, specify line end characters, as well as select the programming language for syntax highlighting (e.g. C/C++, C#, Java, JavaScript, Pascal, MATLAB, Perl, PHP).
Scite has a good response time and works well, without causing the operating system to hang, crash or pop up error dialogs. It runs on a very low amount of CPU and RAM, so it doesn't affect the overall performance. No error dialogs were shown in our tests, and Scite did not hang or crash.