Discovering an annoying computer virus starts with close observation: There are many reasons why a system begins to act funny for no apparent reasons. Those changes in behavior can be the result of a malicious code, but there are other likely explanations as well.

These are some typical virus-induced signs, as well as a few ways to find out whether a virus is causing them.

Degraded Performance

The first thing to consider when your computer is sluggish; is to identify its latest activities. Performance degradation can be the result of a few situations – and a virus is certainly among them. This list provides some clues for making an educated speculation as to why a computer is running slowly:

• Have you made any major changes to the computer recently? For example, have you upgraded to Windows Vista or Windows 7? These newer OSs require a lot more CPU cycles and RAM than their predecessors.
• Have you installed a new program? Newer versions of popular programs like Microsoft Office 2010 and Adobe Photoshop CS5 require a lot more computer resources than their earlier versions.
• Have you downloaded plenty of HDTV movies or other large-size files? Movies and newer games take up hard disk space. If the hard drive is cramped, your computer will certainly run slower.

If you believe that you haven’t made any major changes, then perhaps it is a virus. You will need to check the computer’s behaviors and run a few of simple tests before making a final judgment.

Unexplained activities

Does your network-activity or hard-drive LED flicker continuously for no apparent reason? Of course, there could be a legitimate reason for this, it could also be a sign that a hacker’s back-door program or a virus is active on your system. You could be donating a large part of your system resources to a malicious hacker and be mostly unaware of it. These are a few examples of what might happen if someone has hijacked your computer:

• Someone could be using the computer to send a massive amount of annoying spam emails to thousands of people all over the cyberspace.
• Someone could be using the computer to carry out attacks on corporate computer networks. In a distributed denial of service (DDoS) attack, for example, the hacker instructs many “zombie” PCs (like yours, maybe) to send lots of data packets to a certain corporate system, glutting its traffic and kicking it off the Internet.
• Someone could be using the computer to scan other computer networks, hunting for unprotected ports (communication channels used for specific computer tasks) which can mean gaining more potential victims.
• Someone may have installed spyware to reports back your activities to him or her.  One classic example is a keylogger – a small utility that records all key presses and mouse movements in an attempt to grab your credit-card numbers, bank-account numbers, and other sensitive data that you don’t want anyone to know about.

Hangs or crashes

Does your computer crash frequently? Does it just simply stop responding?  Do you frequently get the BSoD (Blue Screen of Death)? Again, you will find many likely explanations. No cop-out, just plain reality. Hanging, crashing, and blue screens can be virus-induced, but they are probably not. These symptoms may be the result of new drivers, new software, or even a hardware that is about to fail. Check out those possibilities first.

Won’t boot

You’ve guessed it – just because a computer won’t boot, it doesn’t actually mean that it has a virus.  There are many other likely explanations – for instance, a corrupted MBR (master boot record – an area of the hard disk that your computer needs to start up), corrupted OS files or simply loose cables.

If there is a catastrophic damage on your file system, you’d probably need to rebuild your computer’s OS from scratch – not exciting, even for those experts – and retrieving any lost data might get dicey if done in a hurry. However, if you’re a Windows user and have to reinstall the OS, here are two things you should do:

• If you’re using Windows XP or Windows Vista, what better time to upgrade to Windows 7?
• This might be the good opportunity to make a list what you should with the clean OS environment, for example installing an anti virus, recovering as many file as possible and so on.

Strange computer behaviors

Okay, computers occasionally do behave inscrutably, but these behaviors should be easily predictable. Same deal for virus -which means it can’t completely hide their activities.  Perhaps the symptoms are obvious

• Files aren’t where you left them, and after using the Search feature, they can’t be found anywhere. If your system is a Bermuda Triangle that is regularly devouring your files, you could have a virus.
• The files are there, but their date stamp or size is suspiciously different. Virus that infects program files can make the files smaller or bigger than they should be or after some changes change their date stamps. However, changes in file sizes and data stamp can be caused by updates, for example after installing the latest ATI or Nvidia drivers, certain Windows file system may change in size and date stamp.
• A strange message shows up, a few viruses announce their presence by tricking the user. If you’re greeted with a pop-up such as “Your computer is infected with xxx virus! Please visit http://www.xxx.ru to solve this problem”, you probably have a virus that is distributed to aid a scam attempt. The virus intentionally directs you to a shady web site, which will try to convince you to buy their anti virus program at an outrageous price. The app may work, have no effects whatsoever or even make your situation worse.

These are just simple cases of strange things a virus will do to your system. Those virus coders are pretty ingenious, in an ugly sort of way.

Too many pop-ups

A few Internet sites that flood you with unwanted pop-ups might be attempting to inject some malicious code(s) into your system.  Web sites that show pop-ups into visitor’s computers are notorious for trying to change the browser configuration and other computer settings without your permission or knowledge.