How do i get rid of a Trojan Virus? Please Help me!?

How do i get rid of a Trojan Virus, or at least stop it temporarily? Theres a window that pops up that says Microsoft Security Essentials Alert. It says that the virus is one that executes commands from an attacker. It asks if i want to remove the virus and when i try it say it cant be done and it will look online then a different window opens up and its very dangerous and i click okay and then yet another window opens up and says Install Windows Concern System. but nothing happens. When I close that windows the first window just pops up again. I've noticed that whenever this window is open, i can't open any internet browser. it just closes it. I tried to download Spybot Search and Destroy but it closes the browser before I can download it. when i close the window it will immediatly pop back up within 10 seconds. how can i combat this virus? Is there any possible way? The computer is not mine and I can't afford to go out and buy a trojan horse antivirus software. Will someone please help!