How to detect Virus in Your Computer

4 Major Virus Symptoms

• Your system crashes or locks up often.
• It randomly restarts.
• You see strange error messages or pop-up boxes especially alerts about firewalls or viruses.
• Your system suddenly runs much slower, and the key here is suddenly. If it progressively slows down, it's probably getting bogged down by too many programs running in the background, you haven't defragmented it recently (or ever), or you are nearing full capacity on the hard drive. But if the computer suddenly takes minutes to load a web page, or suddenly takes 10 minutes to shutdown/start-up it's a good clue that you have a virus.

Other Virus Clues

• New toolbars in your browser.
• New shortcuts on your desktop that you didn't put there.
• New items in the system tray at the bottom right hand corner of your screen.

Note that new toolbars and shortcuts often come bundled with software you actually want; they may not be malicious — just annoying.

Geeky Tricks for Diagnosis

• If you hit the start button in the lower left corner of your Windows screen and (depending on the Version of Windows you have) either find the run command or just type CMD in the search box, a black box should pop-up. This is a prompt used by computer technicians to run programs at a deep level within the computer and it's often used by the pros to remove viruses. As a result, some malware (meaning viruses, spyware, and other nasty stuff) will disable the feature to make it harder for the user or a tech to remove the virus.
• In a similar fashion, many viruses will prevent the computer from accessing the Windows Update feature, which can also assist in virus removal. So if you go to control panel, Security, and click Windows Update, and your computer can't launch that feature, it's another clear sign you have a virus.

CTRL-ALT-DEL
Some people will tell you to hit CTRL-ALT-DEL to launch the task manager and comb through programs (processes) to look for unfamiliar items. I think this is a bad idea. Virus writers are sneaky and will often name virus files to look like parts of the Windows operating system or other common programs. Conversely, many legitimate parts of Windows have weird names that sound like viruses to a layman.  I have heard horror stories of people thinking they were deleting a virus only to realize after a reboot that they had disabled a mandatory component of the operating system. Then they had a dead computer that needed a complete Windows reinstallation.
But one place where you can get useful info in the task manager is the networking tab. If you have all other programs closed and still notice a high level of network/Internet traffic, it's a good indicator you have a virus. This tip alone is not a good enough metric for virus diagnosis, but it's a clue as you compile information about the state of your computer.

Surefire Virus Diagnosis - Free virus scanners.
There are lots of free virus scanners online that can tell you if you have some form of malware on your computer. Personally, I like to run two or three programs, because there are hundreds of thousands of viruses, all with different heuristics (behavior patterns), so you increase your chances of diagnosis if you use multiple programs with multiple and varied virus definitions.
I like AVG, Avast, and Malwarebytes.  All are highly rated by CNet and PC Magazine.
One tip: when you scan for viruses, be sure to turn off or disable any other security software. These programs do not play well together and can interfere with each other. Also, after you diagnose and remove the virus, keeping one virus scanner is probably a good idea, but you'll want to trash the other two since they can be real resource hogs running in the background.

Virus or Hacked Email
What if people tell you they are getting spam from you? This could either be a virus or that your email account has been hacked.  First try these steps to take back control of your email account. If spam or unauthorized email is still being sent out from your account, it's possible you have a virus. In that case, you need to get rid of the virus on your computer and then go through the process linked above again to change passwords and block access to your email.

Source: http://sg.news.yahoo.com/blogs/upgrade-your-life/does-pc-virus-just-slow-181117610.html