These are some links that I personally find helpful. Please use at your own discretion. Some of the links may be out of date from time to time, so please bare with me as I update them.
If you are getting those annoying popup windows trying to sell you items, drugs, or even pornography, your computer may be infected with "Malware".
The suggestions below are just suggestions, nothing else than that. Please take them as that and take the advice at your own risk. The programs that are going to be cited are free and I personally have used them for years, but neither I, nor the company for which I work is to be held responsible for taking the advice in consideration. I do not give support for those programs either, I just use them.
You can download any of the programs below by clicking on the red link at the beginning of each description.
Please check with your IT person before using them.

Ad-Aware SE Personal Edition: Spyware and Adware remover. Click on the "Download" button below "Ad-Aware Free" to download if you want the free version.

AVG Antivirus: A really good antivirus, and free for personal use. Click on "Download Now" to download.

CWShredder: This program checks for CoolWebSearch, a spyware that is really difficult to remove. If found, it can remove it.

HijackThis: It detects any homepage hijack. It detects general methods used by hijackers to force you onto their sites. It does not target any specific website.

Malwarebytes' Anti-Malware: This program should be used in "Safe Mode" if you have the pests of "Antivirus 2008", "Antivirus 2009", "Personal Antivirus", "Professional Antivirus" or some other fake antivirus that asks you to input your personal information and credit card number, so you can get rid of a zillion viruses that are infecting your PC and not letting you move the mouse or perform any tasks at all. View note below on how to boot in "Safe Mode".

Spybot Search & Destroy: Spyware remover. Choose any of the "Safer-Networking Ltd." links to download.

SpywareBlaster: Checks for Spyware on Internet Explorer and, if found, it can remove it and immunize your system. Click on "Continue Download" to use the free version.

SUPERAntiSpyware: This program may find Spyware that SpywareBlaster and Spybot Search & Destroy may not find.

Sygate Personal Firewall: Free firewall. It may block programs from accessing the Internet if you do not set it correctly. Click on the "Download Latest Version" link on the right to download and use. The installation requires a reboot, so save your documents before installing.

I would run "Ad-Aware SE Personal Edition", "Malwarebytes' Anti-Malware", "Spybot Search & Destroy", "SpywareBlaster" and "SUPERAntiSpyware" at least once a month.
You might have Norton Antivirus or some other antivirus. You could use that instead of AVG.
You might also have a Firewall installed, if you use Norton, or McAfee. That might be used instead of Sygate Personal Firewall.
Remember to update your antivirus, if at all possible, on a daily basis. There are hundreds of new viruses found each week. The best way to protect against them is to have your antivirus up to date.

These are just suggestions. Use them at your own risk.
Note: The PC should be boot into "Safe Mode" by restarting it and, before the Microsoft Windows logo appears, pressing the "F8" key on the keyboard. The arrow keys should be used to select either "Safe Mode" or "Safe Mode with Networking" and then hit the "Enter" key. Once the login screen appears, use the normal password, if any, to log into the PC.