Heloo...

I have had problems with the Custom Client before but this is the worst...

So i log in bla bla press go and the server bla bla....
Thaats normal...
But then when I click on the screen to start mmoving the camera and doing stuff, the cursor doesnt dissapear!!! it goes to the middle of the screen and stays there! I cannot place blocks, or remove blocks, and then a few seconds later the thing goes crazy and starts deleting blocks everywhere!!!
Then I cannot close the Client without pressing Alt+F4 about a hundred times!!

What can I do to solve this???

Windows 7 professional, no admin rights and crappy computer