Since your computer is very weak and you tried to raise FPS by reading How do I raise my FPS?. I also hope that you tried Standalone Flash player that could raise a bit FPS. If you tried all this, only possible (and logic) solution is that you spend a little money. But not for a new PC.
Upgrade your RAM memory, from 2 GB to at least 4 GB of RAM memory
Buy new GPU, not expensive one. After instaling new GPU, turn off built in GPU
These upgrades are less than 80 euros
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I know that you said that you do not have money....but it is the only possible solution to raise FPS. Also it will help you in other PC jobs in the future, not only for playing Tanki.
I do not see any other solution, but that you spend a little money on upgrading.
Good luck