You must install your windows again. But I noticed when I install an antivirus, my PC is slow and I read somewhere this can be a reason, coz the antivirus can make PC to be slow.

I see two variants: first is problem of your computer (viruses, old windows running not as good as you want, wrong presets of operating system and hardware network, browsers, etc) second is problem of your ISP (internet service provider). You need check your computer and if you are sure it's ok (if you don't know computer very good you will maybe need help from people who does, friends from your location for example:) you need call to your ISP and ask him about your problem with low internet speed. But Before I recommend you read your conract with ISP especially about guarantee of internet speed.

AhourA, I know this, but I just wanted to say that an antivirus makes a Pc to work very slow, this can be a reason.

@Irene: That's because the AV search your H.D.D for viruses, trojan horses, malwares, etc. you can disable this option in the settings and it would help ur PC to process faster but it increases the risk of getting infected while you're opening a file on the H.D.D

it depends on the bandwidth that ur isp has guaranteed to share for you, of course, if your PC is infected by malwares or viruses, it affects ur connection speed. you can check ur computer with anti-viruse softwares (make sure that your AV is up2date). for a dial up connection, a download rate of 6-8 kbps is good. you can see ur download rate while you're downloading a file from the net in the IE window.