Hi skruntie
lemme see - james responded to your thread header with the first reply, that is to say reply#1 with
"If the HDD is constantly in use, have you got enough Ram? Maybe it's using the swap file instead?"
This i agree with. Hence i elaborated by adding some tips about making the swap file quicker to respond - so thats where the thrashing/grinding to a halt occurs w.r.t not enough random access memory.
I really do think a regular defrag is a key task in speeding up the machine.
Your problem reminds me of a problem i had a while ago, a cracked copy of a well known security suite, slowed browsing to a halt. I removed it, rebooted, defragged, rebooted, cleared my profile out a bit, rebooted again, defragged again and rebooted. All these reboots are for the sake of "last known good" to be safe. If you reboot between actions then you always have a last known good to fall back on but if you do em all in one go you are asking for trouble

I installed a new security suite and after a few simple settings hey presto ! speedy browsing once again - try it

Sisoft sandra will help with your pc optimising----> for sure

you say you think your problem is software related. . but then you said this "I thinl it was the lack of heat disipation as the hard drive is fitted via plastic rails inside the chassis, rather than being screwed into the metal part of the metal chassis of the case llowing some heat to be drawn away. Ectra fans should not be an issue for hard drives, especially when there a 2 spaces above and below, and the side panel of the case is never ever fitted." - which is certainly hadware related
My comment about liquid cooling kinda finds a home there doesnt it ?
OK I understand your wish to leave making a decision on using a router to later, but know this, a USB adsl modem is a point of presence on the internet, which means your PC is a point of presence on the internet, and as such it has no flak jacket (gateway) to hide behind, and so you are super vulnerable ! With a router, suitably set up + Network address translation, at least you have a chance, along with a personal security suite. My recommendation this week is ESET, with coconut maccaroons and a large bottle of pilsner of your choice.
Lastly i feel for you with your bad 1/2 Tb hdd, ive lost a few hdd myself and its really annoying, but your supplier could have dropped it on the floor, the warehouseman could have dropped it on the floor, and your delivery guy could have thrown it with all his might into the back of the van. All these things happen, so send it back, write a nice letter, and (as a carrot woorks better than the stick) consider yourself a richeous dude because you werent made too angry over this.
