How do you know ?

same as you, when my pc gets slower, I often suspect that it is infected by a virus so I just scan my pc through it's installed anti-virus. It's really good to have a good anti-virus installed in your pc.

if you have an antivirus program see to it that you scan your entire system through it and see what it shows. If it shows some suspicious file then you can suspect, sometimes antivirus might not show, at such situations, either you find some difference in the speed of your system, or sometimes cds or dvds cant be written properly, There will be unusual behavior of your system. then when you notice you must realize that it might have been because of virus and try to remove all the temp files, prefetch and %temp% files. Then you must even check the registry too if you know where to check, check folders if there is any folder which was created without your knowledge, if so delete such folders, all these are the ways we can ensure there is no virus, also we must be very careful while downloading software or games, because many of them contain viruses. Hence... Make sure that you suspect it fast before it destroys your entire system, i couldn suspect virus in my system last time and all of a sudden, 40GB harddisk was affected. Hence, be careful.

I find out as it is happening. my virus scanner lets me know as I try to open the file or before I open it. so I never really get a virus.

Computers, especially laptops, usually don't have enough memory (RAM) to work faster. You should have at least 2 Gigabytes of RAM and also get rid of that huge Nortons anti-virus which most of them come out with and use a free one like AVG.

i rarely uses an antivirus, yeah when PC gets slower,mouse start giving problems, i format my operating system, so i dont care much, today i formatted and now it is flying like a jet,and after a month or so it will slow down due to lots of installed applications and viruses, i ll format then again, thanks

One of the very first symptoms is the virus tries to hide itself. So it disables the task manager so you couldn't end the process which would be silently running in the background. The system gradually slows down and tends to act weird. It might perform actions which you never commanded for.