Need to ask some questions:
1. What exact Mac OS are you using?
2. Have you ever done an disk cleanup/maintenance/repairs, from a software perspective? By that I mean have you ever run any disk-related software to help with those tasks? There are some excellent products available (both free and commercial) that can help you with that.
3. This is critical: are you making backups to an external device? If yo are, what software are you using for that?
4. What third party (ie, non-Apple) software do you have? For those that you have, did you insure that they are compatible with the Mac OS you are running?
5. How much free space do you have on your internal drive?
6. Do any applications launch when you start up your Mac?