System idle process should be at the top of the process tab most of the time. If not, whatever at the top is hogging the cpu. I'd either update that software or find a replacement.
Also, some security software like antivirus can be taxing on the pc. Turn off heuristics (which can cause false positives) & archive scans (malware must be uncompressed to do the work, once it is, it'll be picked up by antivirus). Google cureit & do a full scan. Could be malware.
Finally, do a windows update.