Addons are also "scripts". Anything that is scripted is a script. However, to differentiate between the different sizes of code and how advanced the code is, they are given different names. 'Scripts' that you find on websites in the often-called 'Scripts' section are full scripts that are usually quite advanced. They may change the look and feel of mIRC aswell as provide many features such as mp3 players, away systems and auto-join/identify scripts.
'Addons' do just that - they "add" "on" something to mIRC. It's a bit like downloading a small part of a script - for example, you might download an mp3 player on its own, or an away system on its own - this is an addon.
You can also download "snippets" - these are also scripts, but they are called snippets because usually they are just small bits of code that are often "commented" to explain what the code does. They are used for learning from more than their usefulness, but some can be fun too.
SysReset only alters mIRC, it is not another program nor is it another form of mIRC. So, you don't turn sysreset back into mIRC...you just unload SysReset. I've never used it so I don't know, but try typing //run $mircdir and putting the mIRC.exe file in a correct directory, OR delete all the script files, OR look for documentation that tells you how to unload the script (recommended).
As SysReset is, I believe, well known as a file sharing script, do be aware that mIRC is not a file sharing program primarily. Getting a pic/.wav file off a friend is fine of course, infact it's encouraged. Downloading mp3s, movies and software is illegal, can get you infected and cause huge inconvenience to you and us!
Hope this clears a few things up :-)