my system has gone very slow, plz help

my system has gone very slow it takes alot of time to start any application.i hd downloaded and installed a trial version of norton antivirus some trojons and malwares werte detected and removed , i also defragmented the drives, and increased the virtual memory,. few days back "virusremoval.vbs" was also detected and removed from my pc but wenever i start my pc a msg come "...../virusremoval.vbs not found".
Inspite of all these my pc is still running very slow plz do help.