A common question from users when dealing with computer viruses is how to know if your computer has a virus. Actually this is not a user concern to know if their computer is infected by viruses or not, but only if they had anti virus programs installed. That's why user recommended to at least install a free virus protection programs on their computer to protect system, data, programs or documents from virus infection. But if don't have it or you already used your computer without anti viruses installed before, then you have to track it down by your self based on several traits for the sign of viruses hosted on your computer.
1. Computer become lag or crash very often
Lag and crash can be a sign of viruses running on your system. They usually use some computer resource in the background such as memory, cpu and some also using internet to communicate to their server. Even thought it can be also problems on your hardware and system, but when it come in sudden then you have to warn your self as it could be viruses.
2. Some missing features or tools on Windows
This is definitely things that caused by viruses. If you felt that you didn't remove or delete anything on Windows but you can't find tools such Folder Options, Task Manager, msconfig and regedit from your computer, than that certainly viruses roaming on your computer.
3. Error messages while opening Office documents
This is one of the most annoying virus, especially if you're working with Office documents everyday. Some of them injected them self into document files, some others pretend to be document files while hiding or removing the real documents from hard drive.
4. Accessing usb flash drive taking longer than usual
Most of them spreading them self to another computers though usb flash drive. When you plug in a flash drive into computer and then you see a warning or something suspicious running such as media scanning, or alert for un-formatted media, than you should be aware as it could be an autorun.inf on the drive tells computer to do so which is actually a way for viruses to inject them self into computer.
5. Missing or hidden files/documents on usb flash drive
If your computer infected by viruses, they sometime monitoring in the background waiting for a usb drive to be plugged into computer. When it happen, they usually attach infected files into flash drive by disguising as the files of documents inside the flash drive and then hiding the real files. You can retrieve the hidden files using attrib command. In the other cases files or documents inside usb drive sometime moved or even removed from the flash drive.
6. Can not access safe mode
In order to protect them self, they prevent user to access some windows tool (read section 2) including Windows safe mode. So when you restart computer and hit F8 to choose boot mode, and then select safe mode, you computer will be halted by blue screen of death (BOSD) or sometime it will restarted by it self.
7. Playing with keyboard and Mouse
In some rare cases, viruses also playing with keyboard or Mouse. When you can't control your mouse. like it always pointing to the middle of screen, or it always typing a certain character or can't use keyboard, than it could be malwares playing with your hardware.
8. Slow down or block internet access, block certain website
If you can't use internet like you always be, or internet stats showing high bandwidth usage while you're only opening Facebook, or you can't access certain website like Microsoft or Anti virus website, or you completely can't use internet while your ISP said they're fine, than that could be sign of spywares or Trojan.
9. To many popups
If you suddenly see popups attacking your screen, your computer might be infected by adwares. Adwares usually send popups to your screen while your surfing to internet.
10. Programs stop working or missing system warning on startup
Sometime when you start-up you computer, you'll see several warning of missing system files. This could be due to the files is broken or deleted, or it's only a fake warning send by viruses. This could also caused for some programs or applications stop working.
The most important things is that you can't manually remove the viruses, well.., you actually can, but that's not an easy task since viruses usually spread or evolve them self to many things on computer, for example system files and registry. So you'll need lot of time to track all the files and registry created by viruses. That's why if you sure that the computer is infected, the first thing you have to do is install anti virus programs and let the program do the cleaning job. It is wise to have an anti viruses installed on your PC and updated frequently even if your computer wasn't attacked by viruses.