Computer is running too slow, how will I fix it?

Computer is running too slow, how will I fix it?
When I opened my computer now I noticed that it runs so slow and I don't know what happen.
The last time I used it was okay and running fast. I checked the hardware inside the CPU but I think there is no problem and okay. I check the hard drive if how much free space and it has 20 GB free space. I'm really curious why computer running too slow.
Can someone knows this kind of problem? And how will I fix it?
Please help me.
Thank you very much!
