Do you mean you're going on an overnight / multi-day backpacking trip?
I'd strongly recommend against taking your laptop. It could get wet. Your batteries won't last anyway. You'll be dirty. It will be dirty. It will add a good 6-7 pounds to your pack. If you must bring technology, bring an iPod. Better yet, leave that home and bring a book.
If you're determined to bring a laptop, just get a regular backpack and a rigid sleeve for your laptop. Brenthaven makes good ones. Get a ziploc bag big enough to fit the laptop (they come in extra extra large sizes). DON'T take your laptop out if it's raining.