Ok, for all those people who have Microsoft Windows computers that run slowly and produce pop-up advertisements all the time, I'm going to do you a big favor and tell you what you need to do.  The best part: ALL OF THESE SOLUTIONS ARE FREE!

1)  Check your hard drive

Open a Command Prompt (click Start, Run, and type "cmd" then hit Enter) and run "chkdsk c: /f" to check for file errors on your hard drive.

2)  Run Windows Update

Go to Start, Programs, and click the Windows Update icon.  You'll need to be connected to the Internet for it to work.  Make sure you download all the Critical Updates, regardless of how long it will take!

3)  Install anti-virus sofware (and keep it updated)

If you aren't running anti-virus software, you're asking for trouble.  Likewise, if you think you're running it but haven't updated the signature files lately, you might as well not be running anti-virus sofware at all!

Microsoft provides a free online scanner (OneCare) that you can run manually.  However, you still need to install anti-virus software on your computer for continous protection.  There are some decent free solutions out there like AVG, Avast!, and ComodoAV.

Important: You should only install one anti-virus app!

4)  Install or buy a firewall

First, find out if you need one.  Go to GRC and click the "ShieldsUp!" link.  If it says you have open ports, or you don't even know what a port is, read on...

Hardware firewalls are the best (most internet routers have a NAT firewall), but software versions work too and are generally cheaper and easier to use.  If you're running Windows XP or Vista, it comes with one, you just have to make sure you turn it on.  Note that XP's built-in firewall is only a one-way firewall meaning that it protects your computer from external Internet connections but does not stop software on your computer from connecting to the Internet (phoning home.)

More robust firewalls are available, like ZoneAlarm and Comodo (both free.)

5) Search for ad/spyware

Some anti-virus programs do this already, but just in case, you can download Spybot and/or AdAware.

6) Firefox browser (optional)

If you use Internet Explorer (IE), you might want to consider switching to a different browser.  IE is known for having a lot of bugs, and even today there are still known bugs that have yet to be patched.  Some are quite serious and can put you at risk just by surfing the net!  (Do a Google search on "IE unpatched" for a sample.)

One browser that has gotten quite popular is Mozilla Firefox.  It even has features IE doesn't!  For example, AdBlock with the Filterset.G updater.

You can also download the Thunderbird e-mail client which has spam-blocking built in and is a great replacement for Outlook Express.

7) Backup, backup, backup!

Anyone who has used computers long enough knows the importance of backing up files.  While there are many backup programs out there (Windows even comes with one), restoring a crashed OS where all files were lost can be a nightmare, if not impossible.  Unfortunately this is often the only solution if your system is infected by a virus or your hard drive dies.  One program I use is DriveImage XML.  It backs up the entire partition and allows individual file restores or entire partition restores via a BartPE boot CD.

8) Use a custom hosts file

There is a simple text file on most modern operating systems (Windows, Mac, Linux, etc.) that can be used to both find and block Internet sites by name.  This file is known as a hosts file.  By using a custom hosts file you can block access from your computer to many malicious websites and unwanted advertisements residing on the Internet.  That means that when you're surfing the Internet, advertisements on websites won't be displayed regardless of which browser you're using!  It also means you're less likely to contract malware and, in the unlikely event that you do (or already have), it won't have much chance of communicating with other bad sites on the Internet.  For Windows users, I recommend installing Hostsman to update the hosts file automatically since the list of bad websites is always changing.

9) Defragment your hard drive

Some computers, especially older ones, can start to slow down due to fragmented files on the hard drive.  Windows has a built-in file defragmentor but it's not very effective.  A more effective version is Power Defragmenter.

10) Update your computer's BIOS firmware

Just like the software you load on your computer, there is software that resides on the hardware chips in your computer, present at the time you bought it and put there by the manufacturer.  This software (or firmware) is what makes your computer work and often it can be updated too.  The process for doing this is more complex because you must visit the manufacturer's website and follow their instructions for updating.  For example, if you own a Dell you can download the updated BIOS firmware from their support site.

If you're interested in learning more, visit CERT.