It really is a piece of cake. There are links to disassembling the device that can be accessed through the "documentation" link on the side. Although, if you resoldered your device before, I am sure that you already know all this.
After you have taken your device apart, the worst is over. You need to slide the drive out (I use a mini flathead screwdriver to make things simpler). Then, (carefully!) take off the metal plate that is on the top of the drive. It is best to do this very slowly, as there are adhesive strips that will be reused for sticking the plate onto the new drive. Then, attach the plate to the new drive while being careful not to cover the "breather holes" on it. Finally, slide the drive back in and reassemble your device.
Keep in mind that this is solely the hardware end of it. For your issue, I would leave the Archos disassembled and not put the plate on (or even take it off) until you know for sure if the new drive fixes your problem. Also, it might be wise (and faster) to format the drive and load any files in an enclosure or a 2.5" HD adapter rather than through the player, although there are no barriers to doing it through the player.
Good luck! And, I hope that this is your issue.