Couple of questions, if I may, to clarify some things.
1. When you say the computer lags, are you talking about the time it takes to access programs or games, keyboard lag (time it takes for typed characters to appear on screen), time it takes to come out of sleep or hibernation, or something else?
2. You mentioned using Task Manager to check RAM usage. Did you also check Processes to see if something is hogging the CPU? (Like a disk defrag running in the background while the computer is idle or a virus scan?)
3. Also from Task Manager you could check the Resource Monitor for a more in-depth analysis of what's happening with your computer. Resource Monitor
4. Have you done routine maintenance chores such as defrag, check disk, disk cleanup, etc? This tutorial gives many suggestions on how to optimize Windows 7. Not all will apply to your computer but you might consider trying some of them. Optimize Windows 7