As I just did my HSC last year I can give you the following advice:
You can't rely on programs like these to get you studying, trust me, if you dont have the will power to stop using the computer at your own discretion then you won't, you'll end up just browsing the internet or watching TV etc.
You have to think about what the HSC means to you, depending on your priorites it can be quite important and considering that you're just entering your 3rd term and you're going to be having your Trials in a couple/few weeks you
really need to start studying if you have not done so already.
What you need to do is have something which you want to accomplish during your study time in order to motivate you to study. Going in and opening a random page in your workbook really won't help you at all, you
need to look at your syllabus (especially for the sciences, other subjects are usually not as straight forward but teachers should have explained more or less exactly what's required) and make sure that you can answer every single dot point there. Don't just 'answer' them in your mind as when I do this I tend to be overconfident and think I understand a concept but when I'm told to write it down I realise that I don't understand it quite as well as I expected.
I'm not sure about which subjects you do but most of the subjects I did (quantitative ones such as Maths 3U & 4U, Physics, Chem, Software Design & Dev) don't require you to memorise things as much as they demand you to understand them - especially the sciences. If you're doing any of these quantitative subjects then you need to understand
why something is done in a certain way, simply memorising the formulas or relying on the formula sheets provided is close to useless as you would be
extremely fortunate if they gave you a straight forward question which didn't require any thinking.
To be honest, a lot of teachers say you shouldn't attempt previous exams without studying first, whilst it's true I find that attempting previous tests first (especially in maths) is a great way of understanding what type of things are required of you and is great for setting a goal of what level of knowledge is expected from you. Just make sure you save quite a few for after you've studied up as well
Uhh yeah, that's just my two cents.
Now just a bit about my experience: To be honest I was in the exact situation as yourself, I had real difficulty getting myself to leave the computer alone and go upstairs and study. The most important thing to me was making the decision on any certain day to go up and at least do
something - sure a lot of the time I was just sitting at my desk staring at the wall, but if you set yourself a goal and make yourself DO something (ie. write something) instead of just reading and losing focus after about 2 sentences, then you will definitely learn things and if you can do this for even a few of hours (I found it hard to do more than 3) every day (or at least a few of the days) of the week - which still gives you a decent amount of time to chat on Messenger and watch tv - leading up to the exam then chances are you will be well prepared.
I guess it's really up to you in the end, just remember that you get rewarded for the amount of effort you decide to put in, no more, no less. You know when you're ready to walk into the exam feeling confident and when you need to study more.
Also, remember that after this whole ordeal is over you will have at least FOUR entire MONTHS of holidays before you even need to lift a pen again
I was not a model student at school, I sucked at English (the subject), I was performing below (or around) average in all of my 11th grade subjects, and I didn't spend every minute of every day studying as some people do - I still managed to pull my act together and get a
99.05 UAI 
EDIT: I cant stress this enough: Make sure that you have
something to study or it will be the biggest bore and waste of time ever.
EDIT2: If your computer is in your room I seriously suggest that you get it moved to somewhere where you can still use it for assignments/leisure but that it wont be too much of a temptation while you're actually trying to study
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