Were I you, I might go for Ubuntu 8.04.1 instead, since it's going to have fixes for longer: 3 years instead of 18 months. Also if your processor is 64-bit, you should get the AMD64 version; there aren't any real downsides to going 64-bit anymore and it's a bit faster.
The easiest way to get started is to download the ISO, burn it, eject & re-insert the CD into your drive, and install Ubuntu from inside Windows. You won't have to mess with partitioning and if you don't like it, deinstallation is a snap. Two downsides to this method: drive access is slower, since Ubuntu will be installed to a file living inside Windows' NTFS partition, and your Ubuntu installation may potentially be damaged if Windows has a major hiccup. I don't know if this will prevent you from read/writing your NTFS partition inside Ubuntu (useful if you want to share data back and forth).
Should you want to remove Ubuntu when it's installed this way, you just have to go to Add/Remove Programs and uninstall.
I second the recommendation for Wubi (the Windows sub-installer), but I'd like to speak against choosing 8.04.1 over 8.10. The normal releases are only supported for 18 months, while the LTS are supported for 3/5 years (desktop/server). This support MEANS NOTHING to end users. The newer releases have new features. You're going to want to upgrade to a newer version sometime in that 18 months, I pretty much guarantee it.
The LTS releases DO NOT get updated/new hardware support. They don't get new/updated versions of apps, with a few exceptions. While it's kind of nice to have a stable image to work against, it does NOT matter for end users.
By the way, that 'support' means security patches, not 'call us and tell us problems' support. That type of support is typically provided by the community, which are generally going to be running the newest release.
If you said '8.10 is just released, let it get some patches', I might have agreed. Choosing LTS over regular for most end users is just a bad call, IMO.