The graphics are too high on a PC game I bought?

Don't buy many and I've never had this problem before. Bought Grand Theft Auto 4, installed it and then when I started the game I couldn't see the streets and enviroment around me properly! Is there a way of 'dumbing it down' to suit my computer, if that makes sense. Or is there anything off the good old internet that I can download (for free) to meet the required standard?