You should try to find a working thresh-hold duration beyond which you start experiencing the symptoms mentioned above. You should stop working at computer before that thresh-hold reaches and take rest for a few minutes by keeping your eyes closed.
You should use some lubricating eye drops 3-4 times a day which should help you. Regarding back pain
you can take some analgesics as and when required. You should not indulge in playing computer games so much and take till the problem subsides.