Change that antivirus with some real antivirus like nod32 or similar (check testing certificates in various discipline, given by some publishers and organizations. Dont choose antivirus software by like how name sounds or interface makeup). Avira and AVG cant detect lots of trojans, specially if they trial version, even when detect cant remove before you pay them.
What is temp of cpu? Did zou recently check inside of case, specially cpu heatsink for dust ?
When system idle is 99% and you do not work enything this is ok, that means that pc rest. If you call some program (process) cpu time must be given to that process. When you start other programs situation is the same like with excel ?