First of all, ask yourself "why would anyone want to hack my computer"?

If you can come up with a valid answer, then install a firewall. But if your computer has no significance what so ever, why create problems for yourself.

If your computer is only on when you use it, why would a hacker waste his time waiting for you to turn it on again to resume hacking? It would be a total waste of time and for what? To look at your collection of porn or read your email?

Hackers don't generally hack other peoples personal computers unless they have some sort of beef with them. They go for the government or corporate networks or websites.

What you probably need is a good anti-virus protection program. Worms and Trojans are a bigger threat to you than a hacker.