If you download it from the official forge site, no. It is saying that because the installer is an executable file, and that *could* harm your computer, but only if it is malicious, which forge certainly isn't.
So I downloaded it, and my computer started acting strangely (Minecraft wouldn't launch and I had to restart my computer), so I deleted it and now my computer is running smoothly. Is Forge a large file?
The Meaning of Life, the Universe, and Everything.
tfw a PC gamer
You probably did a bad install. Get a fresh reinstall of your .minecraft folder (this means delete it, backup everything you need: worlds, screenshots, etc.) and reload Minecraft. It will redownload all the files. Reinstall after that.
If you want to play with any mods, FTB or one of the other packs is the best way to do it. Its hard and annoying to do manual mods after 1.6.4 due to the way the launcher works. The FTB launcher does everything for you, is constantly updated, and if you want to play with only 1 or 2 mods, there is a pack called FTB light that is bare bones, and ready for anything you'd want to do.
When 1.9 or 2.0 comes out and we have the plugins feature out, then mods will get easier, untill then, its only going to get harder to manually mod minecraft. This is on purpous, and is working towards getting rid of mods, and stoping messing with the sorce code, and move everything to plugins.
TL;DR If you want to play with mods, download FTB or Tekkit or another pack launcher.
I AM NOT YOUR PERSONAL MINECRAFT MOD SUPPORT AGENT, SO PLEASE DO NOT PM ME ABOUT PROBLEMATIC MODS THAT ARE NOT MINE. If you're having trouble/crashes with a mod, you'll have better luck resolving it in this forum section than PMing me. If you already made a topic, be patient about responses. If you have troubles with anything non-Minecraft related on your PC, I might be able to help, though, but no promises. Even though I could wish to be, I'm not a wizard.
No, it doesn't. Forge is an executable file. It makes changes to the OS like any other executable file. The examples given are from a browser indicating possible harm. This is based on the browser's security settings, nothing to do with Forge.