I have a Compaq Evo W4000 with an 18gig hard drive for the main hard drive and this is not big enough. I've been told there is some kind of proprietary thing on the compaq brand HD's that make only them work with the computer. Can I use an off brand HD to do a fresh install of windows? Or if not does anyone know how to get around any problems there will be with this?

Any off the shelf hard drive should work. Make sure you have the right interface though. I believe for your computer it is an SATA but I'm not 100% sure. Also make sure if you are doing a windows install that you have the CD key for your windows discs. I know that with a seagate hard drive it will come with software to simply move your files over to the new hard drive without having to do a clean sweep and reinstall.

Aaron