what could be the reasons for high memory usage?

Just some of the top memory users there account for 20% of 2 GB on their own. Call it 10% for everything else and double for Super/Prefetching and we get to 60%, in the ball park of where you're at. These are rough estimates, but the point is that your computer is holding a lot of RAM in reserve to load stuff faster. If you start running out of RAM the OS should release some of those resources for other processes to use. 50-75% usage shouldn't be a problem, and empty RAM isn't doing you any good.

Though really, 2 GB is tiny. You should look into a 64 bit OS with at least 4 GB of RAM, if not 6 or 8.

a pc takes a while to completely turn on (loads some things on background), so I'd say it's normal for it to use extra resources in the first 5min it has been on. Yeah, give the laptop a few extra minutes before gaming.

Your prtscreen shows it's using 43% including ~300MB for Firefox & Steam, so the OS is using around 600MB or 30%. It looks good to me :)
If you have a lack of RAM, try completely closing (in the taskbar) Steam, if you need more RAM when playing, some games don't need steam to run. You can try a "Game Booster" if RAM still is an issue, imo these only really help with RAM. Also, if not already, close Firefox when playing games that need the RAM. Slow downs will happen if the game needs more RAM when none is available, as the OS will write unused stuff on RAM to disc (so that the game can use that RAM), and writing to disc is slow.

performance tip: put power settings to High Performance
battery tip: put it back to balanced or less

about slowdowns and noise, try to monitor the temperature while playing (or check the max temp after playing) then post the result here so I can take a look. A program like "Core Temp" or similar should work.

IMPORTANT EDIT: get rid of WINRAR asap LOL. I recommend 7zip, which as a bonus won't be using a whopping 3MB on background :p (joke (partially))

Yeah? 32 bit supports 4gb?

Okay depends on his version https://msdn.microsoft.com/en-us/library/aa366778(VS.85).aspx#physical_memory_limits_windows_7

no need for AutoRuns, wtf. nor to replace Task Manager with something else in 99% of situations
just press "windows button + R", type in "msconfig" go to startup and there it is. you can also take a cautious look at non-microsoft services

I barely use C drive (install everything on D), yet I calculated 100GB to be a good value. I'd say 64 is an absolute minimum, so you might want to take a look at that.