the Vista service pack update forces your PC to rebuild it's superfetch data that had been built up by indexing you hard disk and compiling commonly launched applications. This data is used to preload into what otherwise would be wasted memory programs Vista thinks you are going to launch next thereby decreasing loading times. Incidentally, if more memory is required for running applications some of the data superfetch data is dropped from memory.
In short, your machine should get more snappy the more you use it. – That is the Microsoft theory anyway! 🙂