Removing an infection can be fairly simple. It depends upon a variety of things though, so the difficulty can vary. This could range from removing some simple adware such as a toolbar, to removing a full system infection being hidden by a rootkit.
The first step to removing an infection would be the obvious, run an Anti-Virus scan. If you have a good anti-virus then the chances are it will remove most of the infection. This however is not enough to declare a system all clean. A number of other tools can be used to aid in the removal of malware and I will list some here.
Malwarebytes Anti-Malware - This is an absolutely excellent tool. Malwarebytes is completely free (although a paid version exists) and removes all different types of infections. Malwarebytes is known for it's superior detection and removal and is used worldwide by malware removal specialists. If you believe you're infected then downloading and running a Malwarebytes scan is a must.
ESET Online Scanner - Another great tool. This is from the company 'ESET' who are known for their amazing anti-virus NOD32. This scanner has advanced heuristics and will detect and remove even the most stubborn infections. This tool should always be on the list of aiding to remove infections.
Hijack This - Hijack this is a must have tool for computer professionals. It is easy to use, however please note it should only be used under the advice of professionals. You could damage your system by misuse of this program as it is a very powerful tool.
I hope this will help you if you're infected. Remember, if you're unsure, ask. There are hundreds of thousands of computer forums where you can seek help with the removal of viruses and malware. As a last resort you could contact a professional virus removal company or have a computer shop remove it for you, however remember you will have to pay for this.