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#21
Microsoft Security Essentials for an antivirus, Malwarebytes for anti-malware, Comodo firewall.
Defraggler to defragment your hard drive - it works better than the built-in defragmenter.
Back up your important stuff to a second internal hard drive or an external hard drive. The external hard drive is recommended, as it would make it easier to access your data from another computer should your first drive fail, and you need your data ASAP.
Really, maintaining a computer is easy. People make it out to be harder than it really is. You can set Windows to automatically run your defrags, scans, cleans, etc.. without much problem.
If you don't have a need to keep your system on for long periods of time, then don't. If it's not doing something productive, it's wasting electricity. I keep my machine on 24/7 because most of the time I'm either downloading something, rendering something, etc. It's given me no troubles to date, and everything's still in great condition.
Just remember that you wear your computer out faster if it's on longer, but not by much.