Computer problem - how do i solve?

This computer is doing some weird stuff in the past day. Sometimes pressing a letter key while typing generates random characters on screen and sometimes pressing the keys while typing an email somehow gives commands to open programs, shut down computer. Also MS Publisher freezes when saving if i turn some text a different color. Is there a good "Scan disk" freeware program I can run to fix whatever errors there are? Is there any other actions i can take to fix this? Thanks in advance!