Has it always been like this?
Vista is a beast of a system, and to be honest, the specs of your laptop is a little low for Vista (that doesn't stop manufacturers selling them though). Double the RAM would really help, and is probably quite cheap to buy.
You can scan your system and it will tell you what kind of RAM you need, and what you currently have. Ideally, you want two lots of the same type of RAM, if you are going to do this. If you currently have 1GB RAM on one stick, then you want to buy another stick of 1GB RAM (assuming that there are two slots for RAM in your laptop). If your RAM is current 2 sticks of 512MB RAM (more likely), then you want to buy a kit of 2 sticks of 1GB RAM.
The website will explain all this though.
http://www.crucial.com/uk