Are you using a task management program? Something that closes applications instead of "minimizing" them?
For example Magic Button (free) -
http://www.trancreative.com/mb.aspx
If not, when you run an application, then click the "X" in the upper corner, it doesn't close the application, it just minimizes it, so therefore it still takes up RAM.
Honestly this is not a fault of Dell, HP or anyone else. It's a function of the operating system, and so you will not find a better solution with another vendor....