What steps are needed to remedy a hanging/freezing computer

There is a serious problem that my computer has developed that I am yet to figure out its cause. My computer freezes and I highly suspect that it has a virus. I am stuck because I don’t know what to do next. I am wondering which of the following actions will solve the problem once and for all when I run it.
- Should I run the antivirus to perform a virus scan,
- Have the whole hard disk scanned or
- Should I run a disk defragmenter.
I am also interested to know in what order these actions should be performed that will remedy the problem. Has anyone ever been in a similar situation? Or does anyone have an idea on how to solve this? Thanks.
