What to do if your windows computer is running ridiculously slow (only applies to Microsoft Windows)

So, I get a lot of people coming to me with Windows PC's and laptops which are running ridiculously slow. As I'm fixing one this very moment, I thought I'd document what I have found to be the easiest ways to get them running to their original speed (when they were first purchased).

I will run through it in a chronological order.

1. First things first, you need to be able to get the system to a 'usable' state, meaning you can click on stuff and something happens, and also you can install stuff. Here's how I do it in worst case scenarios:
1. Boot up into safe mode (continuously press F8 when the computer is booting up)
2. Enable the "Windows Installer" in safe mode.
1. Heres a link to an article where I describe how to do this.
3. A part from safe mode, you can always:
1. open the task manager and end processes of anything which is taking up the CPU, but this is a little frustrating.
2. delete startup items in the Start > Programs > Startup menu.
3. Go to  Start > Run -> msconfig and click "disable startups" described here
4. If you're still having problems, run hijackthis
1. It's a very good program and helps you delete entries in Windows linked with Malicious Programs. Here's the home page
2. When you run it you can make a text file. Paste this text file on the home page and it'll analyse it and tell you which things you'll want to remove.
3. Tick the right boxes and bingo.
2. Install and run a free anti-virus.
1. At the moment the top runners for free anti-virus are AVG and AntiVir
2. I have found that  in the past a lot of anti-virus programs are the things that actually slow your machine down. This is one of the reasons AVG and AntiVir have such good ratings.
3. Install and run a free anti-malware application
1. At the moment the top runner is MalwareBytes
4. Install and run TuneUp Utilities 30 day free trial, currently on version 2012.
1. You get a free 30 day trial, but I've found you really only need a 1 hour trial to fix all the problems. Luckily it doesn't "unfix" the problems once it has expired.
2. The main part to run is "one-click-maintanance"
1. It runs through your system and tells you how many things it can 'fix'.
2. Click the fix button
3. Keep running this program until it does not fix anything anymore.

If these basic steps aren't going to fix it, you're best option is to re-install Windows a-fresh and make a nice new start. Possibly consider installing the operating system it came with to begin with? Or even think about another operating system (i.e. not windows). Ubuntu, OS X?

At the end of the day, your computer will at best run at the speed it was running when you first bought it. If you upgrade the operating system you will notice a slow down.

Also, programs which run at startup are going to slow your machine down. I only have 3 running on my startup; DropBox, Carbonite and MAMP. I suggest you really think about how you want to use your computer. It's all about compromise. 

Any suggestions are welcome, especially if it's easier than this list of 'things to do'.