Thanks to those people who replied, the problem is that actually i found what was wrong: i have dllhost.exe and svchost.exe virus and this was slowing down my pc. Although i did check some threads about this virus in the forum. I still can't delete it from my pc. The problem is i run command promt with net stop ['network connections sharing' and 'wins client'), etc.Then i deleted Rpc Patch and Rpc Tftpd subkeys as well as the files dllhost.exe and svchost.exe from system 32. But it still appears. I did download some windows updates but i have them already installed. What should i do?