My DELL INSPIRON 5100 2.8Gig laptop requires 20V DC 4.50A / 90W , I've 3 LiIon battery packs (that are about 4 years old now and I charge up and swap over regularly).
I've just bought a new spare LiIon battery pack off a webshop on Ebay (90 AUD incl shipping beats 280 AUD from a local shop for exactly the same product ... If I can find someone who stocks one that is.)
I have been looking and think something like
http://cgi.ebay.com/Car-Adapter-Fits...2em118Q2el1247 which I am considering buying for my laptop might a goer .... to plug into one of my 4 x 12VDC accessory sockets in the trusty ol' Pajero ,or into one of the two 12VDC accessory sockets on my powertank / jumpstarter pack.
Where I use my gear , there is no access to 220VAC mains and I don't fancy also carting my camping generator about too ....