Computer freezes randomly and gets slow? Happens every couple weeks?

My computer (hp pavilion a720n desktop) Freezes up (wont let me do anything have to hard shutdown) and sometimes goes really slow. It will freeze anywhere from 10 mins of use to like hours or days. This happens for a couple days every couple weeks. Sometimes it wont happen for a month. Its been going on for awhile. Any ideas on what this could be? If there is anything else I should tell you about my PC please let me know. Just starting to figure out computers. Thank you .
-checked for viruses already
-Drivers are up to date to my knowledge (I believe it is automatic updates)