Windows 7: Lags for no reason

I could not figure out what it is. If I left my computer for 30mins, when I came back, it lags. While lagging, I opened Task Manager. The RAM usage its only 2.8Gb out of 8GB. If I use it for 2 mins, it back to normal...what is that?

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

Other Troubleshooting Steps for Windows 7 to work through to check it over good.

So I got into Bios and change all settings to default. After that, I got BSOD in Windows animation screen. I reinstalled my computer and after installing all the important drivers, it happens again. Whenever I want to change the resolution by right click desktop and open "screen resolution", In there the drop down menus are acting weird. It drops when I click it, but it went back up after clicking. And the Intel graphic control panel, when I open it and my mouse is hover over the windows, it kept switching from the pointer to loading...

Run the hardware tests in Troubleshooting Steps for Windows 7.

Did you use only the drivers provided by the installer and optional Windows Updates after enabling Automatically deliver drivers via Windows Update (Step 3)?

The only drivers which should be imported are those still missing in Device Manager after all Windows Updates are done.

If you didn't follow these Best Practices from Clean Reinstall - Factory OEM Windows 7 then I would start over and do it correctly. This time wipe the HD first with Diskpart Clean Command accessing DISKPART At PC Startup.

Don't install anything other than all Important and Optional Updates, to see if the problem occurs. Then provide any missing drivers one-by-one with time in between each to check performance. Do not reinstall any driver bloatware packages beyond just the driver itself. Report back on performance, any functionality which is missing.