Scam message - HELP

[adomac]

I have just downloaded LimeWire and got a message to download an urgent Security upgrade, so did so. I later looked around the program and came to FAQ where someone else had got the same message and asked if they should download it. The answer was DO NOT DOWNLOAD AS IT IS A SCAM AND CAN DAMAGE YOUR COMPUTER.

So now what do I do? Is there any way to get rid of whatever is now inside my MacBook? In how much danger am I? How do I protect myself? Bear in mind I am no expert, just a 68 year old user a bit out of my depth and very worried indeed.

Very grateful for swift advice.