I would say that the behaviour shield, if it is performing in the same way as the AVG version is going to be a powerful defence. When it isn't in use, it won't be consuming resources.
Me, I would be looking at:
1. is it possible to upgrade the RAM on this system.
2. look at the many programs that when installed want to run on boot, when they don't need to be run until needed.
3. Microsoft Windows managers RAM, after all it is there to be used. When it is running low it takes it back from other programs when it is needed by by an active program.
By far the best option is to get more RAM if that is possible.