To computer expert : why is my computer slow?

hi to all...lately my computer seemed really slow.its takes more than 5 min at startup.i ran a virus scanning , it got some worms and kits deleted them,and disabled some of the programs at startup but its still slow. what could be the cause?apart from my hard disk being full ( on is 15% empty and the other onle... show more hi to all...lately my computer seemed really slow.its takes more than 5 min at startup.i ran a virus scanning , it got some worms and kits deleted them,and disabled some of the programs at startup but its still slow.
what could be the cause?apart from my hard disk being full ( on is 15% empty and the other onle 7% empty)..
and also i can hear the fan revv at really fast speed , it makes a noisy 'vooooo' sound , whereas when i bought it , i could not even hear the fan !it means something is over heating..

any of you can give me tips?could it be the BIOS battery? its been low lately?
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