IMac freezing intermittently--every few minutes. How can I fix this?
It's very unlikely you downloaded something or broke everything with one download. Also - with at least 4 things not working well - please protect your pictures, emails just in case it's a hardware failure.

When only one thing is broken - it's often worthwhile to poke, learn, understand what broke and look for a fix of that one issue.

For any group of three or more problems, there's one solution that always works (and often saves a lot of time):


Back up anything you can't afford to loose and get someone who is a bit tech savvy to verify you have a workable backup that you can restore if needed.
Erase the mac and install just a clean system from the media that came with the mac. 


This will quickly let you know if the hardware has failed. It will also solve nearly all software issues. Once you have surfed the web, made sure it is reliable, then you can run updates, restore your personal files and get on knowing it was just some corruption of the software.

Don't feel bad getting help from someone (even paid) if your savvy is for things other than a computer. A good tech can erase and install Mac OS X in 10 to 30 minutes. A good tech might take 4 hours to 4 days to pick apart all the things that could be wrong and tell you what happened. With multiple issues, it's hard to tell what was the root cause and what was just a subsequent failure.

It's often so much easier and efficient to just clean up the mess, start fresh, and keep an eye for any sign of the problems recurring. You will then have better data on the problem. You will also have a backup of your important files.

Since your computer can't tell if it has enough memory to run, (or is truly out of space) you can't really trust itself to fix itself which is why I jumped right to backup / restore for anyone on a situation as you describe. Also, if you were to determine exactly what broke, and did the fixes right, if it freezes or doesn't complete your fix - you will still be in the same badly working situation.

Best of luck - some good people have made answers with great advice here as to what could be wrong. I wanted to weigh in with a different sort of advice since no-one (you included) had mentioned protecting your data files.