Are you using Vista and very curious where your memory is going? The culprit is the service Superfetch, it keeps track of your computing behavior and preloads the app’s you are most likely to load at certain times into memory. It looks like you have some leaky application, but is is a part of the operating system that is doing this on purpose. It sounds like a good feature, but the implementation is not done well afaik: this process should give up his space when the user needs the memory. This is at this moment not the case.
As a trainer that uses a lot of memory for his virtual environment, I disabled this service. In the next week I will see how it inflicts my performance. I do hope I will not see my memory leaking away. I do not use a lot of app’s, so prefetching them is no use (to me)