Can anyone help me?????

let me start by saying i am not computer smart...i just know enough to get me by...i have gotten a virus or trojan somewhere that has shut my computer down...it is some kind of ransom virus or trojan..i have no idea about the name of the virus....my computer comes on normal, loads the desktop up fine then all of a sudden the screen darkens and these words come up on screen......your computer is blocked!!!!! All your files were encrypted with a strong algorithm and there is no chance at all to decrypt them without the decryption password.If ou want to continue to use your computer and your files,you have to buy the unlocking and the decryption passwords.Just send us $300 through western union and your computer is back.....it goes on to say send the money and we will send you the passwords after we pick the money up.....i cannot do anything at all on the screen including turn it off...i am in safe mode now and i have run 4 or 5 programs to no avail...i have a sony computer and windows vista home edition.....i had a virus like 3 months ago that cost me 220 and my computer gone a week and the dude wanted 340 at first and i had already told him i was unemployed...when i took it in he said it would be a couple of hours and 150 dollars...i will never go there again....he put all these free protection programs on here and you see what happened...can anyone help me out as i cannot afford taking it anywhere....thanks