While computer security programs aren't perfect, they're still a critical part of keeping your computer safe. Make sure you have a good one. And keep in mind that its protection is not absolute.  There are always new programs that have not been detected by the anti-virus or anti-spyware companies.

Use an online scanner: If your software isn't updating and you have other signs of an infection--like adware programs that seem to come back instantly after you remove them--turn to one of the free online virus scanners listed below:

Is your computer safe from online threats? Security check will provide a free online scan.

Acunetix
Web site vulnerability scan to ensure your web site is safe.  Hackers are concentrating their efforts on attacking applications in your website: 75% of cyber attacks are launched on shopping carts, forms, login pages, dynamic content etc. Web applications are accessible 24 hours a day, 7 days a week and control sensitive data such as customer details, credit card numbers and proprietary corporate data.

Panda Software

Scans, disinfects and eliminates over 110,000 viruses, worms and Trojans from all system devices, hard disks, compressed file and all your email. Detects spyware. 84 percent of malware installed on computers worldwide is spyware.

Online virus detection scan

Trend Micro

Ease your mind and scan your PC for viruses and spyware.

BitDefender

BitDefender Online Scanner is a fully functional antivirus product. It features all required elements for thorough antivirus scanning and effective cleaning: it scans your system's memory, all files, folders and drives' boot sectors, providing you with the option to automatically clean the infected files.

Malicious Software Removal Tool

Trend Micro

Trend Micro CWShredder is the premier tool to find and remove traces of CoolWebSearch – the name for a wide range of insidious browser hijackers– from your PC. 

Sygate - Quick Scan

A quick and accurate scan of your computer to detect open, closed, and blocked ports.

Sygate - Trajan Scan

Scans for active trojan horse programs throughout all 65535 TCP ports.

Kaspersky

If you discover a suspicious file on your machine, or suspect that a program you downloaded from the Internet might be malicious, you can check the files here.