Computer access is too slow? How to solve?

Users questions:I newly installed system, C disk space has 2 G, there should be no viruses, there are software Norton Internet can be a very high CPU utilization reached, even to 100%. Open the page very slow 7,777,771, if the virus anti-virus software with full kill on a 2, minnap what? Listen to music you? I am a novice ... ... 3, may be the anti-virus software to monitor and make open the pdf and so after going to the scan, the solution is to set the antivirus software for the general level, not a dangerous level.
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