Alright for real. You can play everything from Atari to Xbox on your PC. All you need is an emulator and some roms. What's that you say? You don't know what an emulator is? Let me explain.
An emulator is a program that runs on your computer and can interpret a game file (rom). You can get any type of emulator you want by going to The Emulator Zone. On the left hand side of the site you can pick out which gaming system you want. You simply download the emulator and then run it. Within the emulator you will have to open a rom (usually by going to File>Open). My personal favorites are Zelda on n64 and Commander Keen on DOS.
Now for how to get the ROMs. The easiest way is to use torrent sites or a bitTorrent client like Vuze. For example in Vuze you can just do a search for "zelda ocarina of time" and you will probably come up with a rom.
You can also just get the roms off the web, but that has been very tricky in my experience. It is illegal for a website to host many roms on it, so it can take some work. But just start by using google. Say you want to download Zelda for n64. Do a search for "download n64 roms" and you will get plenty of results. You can usually find what your looking for within the first three results.
Once you get to a site, some of them require you to "vote" for their site a number of times before you can access their roms. At first I would skip these types of sites because I thought they were scams, but out of frustration I tried one and they actually are legit sites, and usually have the best selection of roms.
Most of the rom files have strange symbols in them, so i'll give you a list of the most common symbols and what they mean.
[a] Alternate [p] Pirate [b] Bad Dump [t] Trained [f] Fixed [T-] OldTranslation [T+] NewerTranslation [h] Hack (-) Unknown Year [o] Overdump [!] Verified Good Dump (M#) Multilanguage (# of Languages) (###) Checksum (??k) ROM Size ZZZ_ Unclassified (Unl) Unlicensed
So basically, you want to find the ones that have [!] in them, and avoid ones with [b]. Good luck gaming! Let me know if you have any questions, comments, or suggestions!