[adamcharding]

Hi to everyone who reads this, and thank you in advance for any help I am given. It really is appreciated.
My problem is essentially that my computer as been running very slow for a few months now. It hasn't bothered me too much as I have been preferring to use my macbook most of the time lately and so I have just been staying away from it for a bit. However, there's not much point having a computer I lament using, and as I would now like to use the extra processing power for more demanding tasks, the laggyness is becoming something more than a simple annoyance.
The way it affects the use covers everything, from video playback (even youtube videos) to merely moving the mouse around the screen. I should mention that these are all processes that had run perfectly for a year or so since I built the computer and so the issue has been a progressive one. Also, the video playback isn't an internet issue, it occurs on every video, even fully buffered or local ones, presenting itself by stuttering the audio and image.
The problem only worsens when more intensive applications are open (photoshop, after effects, etc) and makes it very difficult to be productive as every process seems slowed.
Now I could understand this appearing cut and dry, I thought it was, but on inspecting, the lag occurs when there are no signs that the computer is under any kind of strain. I have made an active effort to migrate non-essential files from my main hard drive to external or additional physical drives so that my boot drive is running with around 150 GB used of its 500 GB capacity. I have also cleaned up the startup files so I have as little running from boot up as possible (I made sure I wasn't taking away anything that would affect Windows running smoothly, besides, the lag was already occurring at this stage) and now I have only the bare essential processes running in the background. Yet, when I have no additional programs open, the mouse still can't make it from one side of the screen to the other without lagging half a dozen times. My main gripe with this is that when monitoring the performance of the system when there is nothing else running, the RAM is only using about 1.5 gig of 4 (mainly OS I'm assuming) and the CPU is running at around 10-15% capacity. And still I can't move the mouse smoothly.
I have also reinstalled and updated the mouse driver as well as switched its usb port, just in case. It made no difference, though this wouldn't have solved the system wide lag anyway.
Sorry for the long winded explanation, but I'm pretty frustrated with the whole situation and I've more or less run out of ideas. Now I'm starting to think that maybe the hard drive could be ready to give up the ghost, but I really don't want to chuck any more money at it without getting some second and third opinions. So this is where I hand off to anyone who could possibly help me, and solve my lag misery.

The specs of my computer are:

Motherboard: Asus P5Q SE PLUS iP45
CPU: Intel Core 2 Quad Q8300 2.5GHz
RAM: OCZ 4GB (2x2GB) DDR2 800Mhz PC2-6400
Graphics Card: Nvidia GeForce 7600 GT
PSU: Corsair 650W TX Series

Thank again in advance for any help.

Adam

[Oddba11]

Same symptoms if you boot in Safe Mode? Likely not...

Just to cover the basics:

1) using your AV software, perform a full system scan
2) perform a malware scan using a tool such as Malware Bytes

after everything is clean:

3) ensure Windows is running the latest SP, and has all of the latest updates/patches

4) manually update/reinstall all of the motherboard/system drivers (rebooting as needed)

5) manually update/reinstall the video drivers (rebooting as needed)

6) manually update/reinstall all other hardware drivers (such as audio)

reboot and test again