Auto Clicker is a term normally used to denote a software which can do some sort of automated mouse clicks. Without downloading there is no way by which a software can do automatic clicking of mouse cursor. Auto Clicker without download is virtually impossible to have as Windows security does not allows any piece of software running from browser to take control of physical mouse clicks. Yes you can watch a video without downloading or you can even listen to music without downloading audio or video files. On a system level, whatever you view on browsers or anything else is actually downloaded on your computer and then the appropriate software presents the information in which it is supposed to be displayed.
In case you are looking forward to get an auto clicker without download, you might be worried about viruses or other malware which various software applications are known to install on your computer and hence the genuine concern. Websites are made up with high level programming languages such as ASP, PHP , Java Script, etc. These high level languages when run inside a browser can do lots of stuff but they cannot take over your windows computer. Every new version of browser tries to add up to the security so that no malware or virus actually gets installed onto your windows computer. Most of the viruses / malwares distributed using webpages actually trick user to actually install a .exe application onto the windows computer. The .exe files are the application files on a windows computer and they do have access to almost everything on a windows computer, controlled with security settings. Java applications on the other hand are not application files and instead they are run by a Java Run Time Environment (An application which must be installed on your Windows Computer), similar concept is followed by .NET applications and hence there are less chances of a downloaded auto clicker to contain virus or other type of malware. Java or .NET applications are often distributed with a setup.exe file (which of-course requires you to download it) which is an application and can be affected by virus or malware. Java Applets are also application and broadly they do not require you to download an application, but they have limited access and cannot take control over your mouse or keyboard.
Desktop application like an auto clicker can be really simple or sophisticated. Auto Clickers are not only for clicking but are used to create scripts / macros to automate multiple mouse clicks. Such Mouse Automation Applications are also many times supported with automatic typing on keyboard, automation of other windows aspects such as automatic application launching, text to speech automation and other lots of stuff which only an application file can do in the best possible way. Ofcourse you might not be very much interested in struggling with complex application switches, command line parameters and other related stuff.
Taking into consideration all the technical complexities, what is the best possible way to get an Auto Clicker without Virus and possible without downloading anything on your Windows Computer ? The rest of this post tries to provide summary of all technical stuff above. Auto Clicker without download and runnable in a browser is impossible to have irrespective of the programming language used to create them. On other other hand you can get an Auto Clicker without Virus or malware and automate your windows computer as you want to do. Whenever you download an application from internet, save it to your computer, scan it using online virus scanner(s) and when found clean, only then run the application on your windows computer. Software vendors often digitally sign their application utilities to make sure that users do get a clean application.