LifeHacker have put together a guide to dual-booting Windows 7 and Ubuntu:

What you'll need:
 * Windows 7 installation disc: For clean installations, either a full installation copy or an upgrade disc is needed. If you own an upgrade disc but want to start from scratch, there's a way to do a clean install with an upgrade disc, though that's a rather gray-area route. Then again, there's probably not a person on this earth that doesn't have a licensed copy of XP or Vista somewhere in their past.
 * Ubuntu 9.10 installation image: You can grab an ISO at Ubuntu.com, or hit "Alternative download options" to reveal a (usually very fast) BitTorrent link. You'll want to get the ubuntu-9.10-desktop-i386.iso download for 32-bit systems, or ubuntu-9.10-desktop-amd64.iso.torrent for 64-bit on AMD or Intel systems (despite the name).
 * Blank CD or empty USB drive: You'll need one of these for burning the Ubuntu ISO, or loading it for USB boot. If you're going the thumb drive route, grab UNetBootin for Windows or Linux, plug in your USB drive, and load it with the downloaded ISO image.
 * All your data backed up: Even if you're pulling this off with Windows 7 already installed and your media and documents present, you'll want to have a fallback in case things go awry. Which they shouldn't, but, naturally, you never know.
 * Free time: I'd reckon it takes about 2 hours to pull off two OS installs on a clean system; more if you've got a lot of data to move around.