I've used openoffice without a net connection lots of times. I think I know the product you're thinking of, but open office definitely works offline.
Go to http://www.ubuntu.com/getubuntu/download and download a disk image and burn it to a cd, or just order the free cd. Boot off the CD and play around a bit. If you like it then partition a drive and install it and play around more. Leave XP on your other partition so you can go back when you need to.
I started using Ubuntu when I inherited a laptop with a dead harddrive. I didn't want to buy a new drive until I was sure everything else worked so I booted off the Ubuntu disk image to test it out. The disk image has Firefox, Openoffice, and a few other programs already installed so I was able to work and just save files to a USB key. I did that for about a month before I finally bought a hardrive.
I still have trouble getting my wireless to work with it, and I have a Tascam USB interface that won't work with it unless I hack the firmware, which is why I still have Mac and Windows systems. Every 6 months there's a major update to Ubuntu so I'm just waiting for them to include the last couple of drivers I need. Everything else was completely intuitive. If you want to install software you just click on add/remove then browse through the list of available software until you find what you want, click on it, and it will download and install itself. 90% of everything I've needed has been completely idiot proof, and the other 10% has been next to impossible.