Your CPU barely exceeds the minimum system requirements for the game. Sometimes developers mess up with the minimum requirements, so it's not a good idea to run a game on a system that is right on the edge, like yours. You also don't have enough RAM. Even if you are on Windows XP, 1GB simply isn't enough for gaming anymore. If you have Service Pack 3, XP starts to run very slow with only 1GB of RAM.
Seriously speaking, you are probably going to need to get a new system to run Mass Effect or any other modern game. A relatively low clocked Prescott simply can't keep up with any modern title.