Acer Aspire 5742G performance issues (freezing for a few seconds)

hi everybody this is my first post in the forum and i'd like to thank you in advance for the support you've given around here.

now, about my specific problem:
i have an Acer Aspire 5742G laptop. i'll paste the technical specifications in the end of this post.

I've been experiencing serious freezing for a few seconds in games, but something's a little off; it only happens when something "new" happens during the game, for example, when i reach a checkpoint or in the beginning of a scripted event (it happens in Torchlight, when I kill a mob and it drops some loot; it happens in Killing Floor when i switch to a new weapon, when it changes rounds and when there's a grenade explosion).

I've been playing Torchlight, Killing Floor and Sniper Ghost Warrior and get this issue a lot of times and with different games, so I don't think it's game related. I do notice my hard-drive starts to spin faster during the freeze and when it reaches max speed, the freeze goes away. I also get this problem when browsing or doing some minor tasks.

I have the latest graphics drivers, I've already changed from Eset Smart Security to Microsoft Security Essentials (which i've been told it's less resource-demanding)... I don't know what else to do.
It's as is the hdd is "dead" when nothing new happens, and then it wakes up, causing the freeze during the period it takes to wake up...
i'm thinking i could use some sort of software to keep the hdd activity up to like 30% or something just to keep it from dropping to low speeds.
There's also the issue with the graphics card, but I think it's taken care of. This laptop has Nvidia Optimus technology, but I did solve it with the latest graphics drivers.
What's your take on this?

the specs:
processor: Intel i5 480m 2.66GHz
hdd: 750GB
graphics card: nvidia geforce gt 540m / intel HD
memory: 8GB DDR3 RAM

i can't think of other hardware specifics. if there's anything else you'd like to know about the system, let me know.

EDIT: forgot to mention i'm using Windows 7 Enterprise SP1 64bits.
also i've pulled some info about the hdd:

Drive Model: WDC WD7500BPVT-22HXZT1
Media Rotation: 5400 RPM

 

Hi.Welcome to NBR.

Prime suspect would be WD HDD and its head-parking policy.
Use QuietHDD for testing and disable APM.

 

thanks for the quick response! i'll do some testing with QuietHDD. let's hope for the best

EDIT: i think it's working! thank you so much. this just saved me the trouble of sending the laptop for repair and it would come the same way since they'd probably just format the hdd. again, thank you!