My son bought an Archos 404 two years ago. Within a few months the player swelled without explanation. Unfortunately he didn’t really tell me about it for several months. When I looked at it,
I discovered that battery swelling was common, but we were under the impression the company wouldn’t do anything because it was out of warranty.
I was wrong. They would have replaced the battery, but I had already installed a replacement.
The good side, shipping charges would have cost as much as the replacement. I ordered from www.newmp3technology.com . After only a month the Li-PL battery was unable to hold more than 5 minutes of charge. But I couldn’t be sure whether it was because my son insisted that it didn’t need a full charge before the first use or over discharged the battery or the battery was inferior.
I then ordered a new one from www.megacapacity.com . It arrived in a week or so. It was rated with a few more milliamps capacity than the original. I was insisting on maintaining control of the Archos until full charge was reached. It’s been about a month and its doing well with charging. Check the web for tips for how to prevent over discharge.
Short Tutorial to replace the battery:  Unscrew the two screws on the control button end of the Archos 404.  Gently lift the panel from that end. The other end has locking tabs. Be careful not to bend them.
 In the back you’ll see two things filling up the space. One is the hard drive identifiable by it’s squarish shape, the copper and ribbon connector and the circular core of the hard drive. The battery is the other and is rectangular with a plastic covering. You may or may not see clear markings with amperage and voltage.  Gently lift the battery. It may be adhered in place with a double sided tape. Pry slowly and carefully. When the battery lifts up, be careful to keep the battery leads intact that lead to a tiny connector under the hard drive.
 To install a new battery, basically reverse the process. Take care with the battery leads to ensure they don’t take up space inside the case.and make sure the lid seats well.  Very Important! Follow the directions for recharging and charge for the FULL minimum time period. However I have one recommendation. Use a powered USB hub instead of your PC ports. Powered USB hubs generally put out more amperage than a typical PC port which means faster and more complete charging. This also means the Archos doesn’t have to be connected to your PC to recharge.