Virus Attack Help !

Hi


I am newbie , not much idea how to dealt with virus attacks. I am facing the following problem :


Not able to access internet.


Processing speed very slow.


I had installed avast and before that mcafee was there , the virus hit i generally saw were trojan and hitting the system32 folder. Can you please suggest me what to do and which antivirus i should use.


Also once i did reinstallation of OS but first time i opned the gmail after that again had the virus hit.


i dont know what to do. Can anybody please help !!