Probably the feature from Vista, use all available unused RAM for loading stuff for a better performance. This is great and the RAM is released once needed by a program. You can use "Performance Monitor" or the "Task Manager" to see if you have a problem with a particular application which use to much.
This is odd, Widows 7 64 only uses 1500 of my RAM installed and have 4500 left to play with. Never seen my RAM usages hit over 2500 when installing softwares. Could be the type of RAM you are using, the Cas Latency of the RAM, or the number of programs you are running at the same time.
You should also look into loneeagle's suggestion as well.