Welcome, Newcomer. If you're using Vista/7, press Ctrl+Shift+Esc to bring up the Task Manager. Click on the Performance tab. Click on the Resource Monitor button.
Find out what the biggest RAM consuming app is.
90% RAM usage is very high. At idle, Windows uses ~5-10%, depending on things like the version of OS; pre-installed software (from prebuilt systems); system services; etc. Given this scenario, a RAM upgrade might be in order, but if you're using a 32-bit OS, 3GB - 3.75GB is the max usable.