Lags And FPS Drops When Playing All Various Games On PC?

Recently, I have reinstalled my OS Windows 7 due to Windows freezing on Welcome screen. Why it was freezing on Welcome screen I don't know, but I have already fixed this problem, so this isn't my main problem. I want to note that before reinstalling Windows all my games were running perfectly and smoothly without lagging or freezing. But now, when my OS is reinstalled, games run improperly and with lags. Yes, this problem isn't weird at all, but the strangest thing is that I can make games run smooth and without lags, BUT NOT PERMANENTLY! Let's say that I want to play Sniper Elite V2. I turn on my PC, start Sniper Elite V2 and doesn't matter on, which settings or resolution I'm playing, it still lags. Okay, I exit from the game and go to this game's shortcut properties, then I check the box and set the compatibility mode for Windows XP ( Service Pack 3 ) and start the game one more time. The game runs smoothly, without lags on maximum settings and resolution! The problem solved and everything will be fine... NOT AT ALL! I turn off or restart my PC and start Sniper Elite V2 again. Bump! The game heavily lags and no matter, which resolution or settings I set, it still lags and runs improperly. Then I go to game's shortcut properties again and uncheck the compatibility mode box. I start the game and it is running smoothly, without lags on maximum resolution again. I restart or turn off my PC, start the game and it lags again! This problem is like a tale without the ending! Now, let's say that I want to play TES V: Skyrim. I start the game and it lags like all other my games. Compatibility mode method doesn't work on TES V : Skyrim, so when the game is running I press Alt+Tab and start Task Manager. Then I go to the Processes tab and then find the game process. I set the priority of this game to low and get back to the game. It doesn't lag on maximum settings. Okay, then I want to play this game instantly without lagging, so I find the code which lets me to run the game with low priority instantly ( %comspec% /c start /low AppName.exe ). I do the thing which I needed to run a game in a low priority and then start a game. Bump again! It heavily lags! While running TES V : Skyrim game, I Alt+Tab and start Task Manager, then I go to processes and see that the game is running on a low priority, but still lags. So, I change the game's priority from Low to Normal and get back to the game. IT DOESN'T LAG AND THERE IS NO FPS DROP! Yes, it is a strange thing... But that's not all. I know a method which allows me to play games without lags or FPS drops. I go to Msconfig > Boot > Advanced options and check the maximum memory box. This allows me to play all my games without lag or FPS drop, except The Elder Scrolls V : Skyrim ( but it isn't very important that I can't play TES V : Skyrim without lag with this method ). I can play my games without lag after PC restart or startup, when the maximum memory box is checked. Yes, but then appears other weird problem. When the maximum memory box in the msconfig is checked, I go to the My Computer properties and see that there is a problem with my RAM. Total installed memory is 4.00 GB, but only just 3.46 GB are usable!

1. Why I can't play games normal, like I played when I had previous Windows 7 OS? ( Note that on previous Windows 7 most of my PC games were running smoothly and without lags on maximum settings and resolution )

2. Why, when the maximum memory box in Msconfig is checked, my PC uses not all installed RAM (3.46 GB) ? ( Note that, when the maximum memory box in Msconfig is unchecked, my PC uses all installed RAM [4.00 GB] )

3. Is it possible to fix these problems? (Play games without lags, like I did on my previous Windows 7 OS [or] play the games without lags when the maximum memory box in msconfig is checked and make (force) my PC use all my installed 4.00 GB RAM (Not 3.46 GB!).

My computer specifications:

Operating system: MS Windows 7 Ultimate 64-bit SP1
CPU: Intel Core i5 760 @ 2.80GHz (Lynnfield 45nm Technology)
RAM: 4.0GB Dual-Channel DDR3 @ 665MHz (9-9-9-24)
Graphics card: NVIDIA GeForce GTX 460
Monitor: SDM-HX73 (1280x1024@60Hz)
Motherborad: Gigabyte Technology Co., Ltd. H55M-S2H (Socket 1156)
Hard drive: 977GB Seagate ST31000528AS ATA Device (SATA)
Sound card: Realtek High Definition Audio
All my drivers are updated.