If the heat-sink and fans are the same ones as when the PC was new (ie haven't been replaced with ones not up to the job), it could simply be a case of heavy dust-accumulation inside the system box. Disconnect from mains and carefully investigate. Check that all fans are working by plugging the PC back in and switching on for a few seconds, as sometimes a fan can fail, they don't last forever. Carefully use a vacuum-cleaner with a plastic crevice-tool attached to remove dust and gunge from the air-intake/exhaust grills and from around the fan casings (a plastic crevice-tool remember -- don't touch computer components with metal attachments). Replace any broken fans with a new fan of same or higher performance. Re-assemble PC and test temperatures again, compare readings with what you were getting before.