Low on Memory when memory is available

I have a desktop with 2 GB Inter Core Duo processor. 2 GB ram and 180 GB hard drive. I am using Windows 7 as my operating system. There are 2 things happening on frequent basis. When I boot my system it gets stuck on the first screen for about 5 minutes and then when I start an application it gives me a pop up (Low on Memory). As I never use heavy applications and when I check my memory usage it is also available up to 70%. I have tried different remedies but still I am unable to overcome this problem.
