How to detect/remove a computer virus?

I was often asked how can I find out if I've got a computer virus.Most people don't know they've got the virus and they keep doing their usual stuff,till one day when everything goes crashing down.How can you detect a virus that has installed itself in you system?
This guide will try to help you detect and remove an eventually virus/malware/spyware from your system.

1. How to check if your computer is infected

2. How to investigate files identified by your AV as malicious

3. Simple programs that can remove most infections

4. How to remove files that are locked and unable to be deleted

5. How to create and use an AV Rescue Disk

6. The importance of Host-Based Intrustion Prevention Systems (HIPS) and which to use

7. Free Antivirus

8. System Virtualization

9. Internet Addons for Security

1. How to check if your computer is infected?
If you think your computer may be infected, but you're not sure, please perform scans with these programs. Between the two of them they should only take about 10 minutes to scan. If your internet connection is down then neither of these programs will be able to work.

I would first use Comodo Cloud Scanner
This program does not have the capability to clean an infection, but it does do a great job of letting you know if you are infected. If you choose you can use the free 30 day trial of Comodo livePC Support that installs with it to have Comodo experts clean the computer for you. Just for your information, installing Comodo Cloud Scanner also installs the program for Comodo livePC Support. It is not dangerous, but you can safely uninstall it if you don't want to use the free trial. After clicking 'Scan Now!' you really only need to look at the malware and suspicious files section. Under this section click the 'Submit All' button and it will send all the unknown running files and processes to CIMA to be analyzed. These results should allow you to tell whether your computer is infected or not. Please be aware that this program is relatively prone to false positives. If it identifies a file as malicious please check it.

I would next do a scan with Hitman Pro
This scan will only take a few minutes, but scans your computer with 5 major security products. It comes with the ability to remove malicious files for the first 30 days after you first use it to remove a file and after that will only alert you to their existence. Hitman Pro only scans the most important parts of your computer, but it does not need to be updated as it checks files ‘in the cloud’. The option to 'Scan computer daily during startup' can be disabled if you like.

If neither of them finds any suspicious files it is very unlikely that you are infected. However, even if they do it is still possible that they are false positives. So check the files again.