Desktop : XP Pro SP3(rc1) due to being a gaming machine necessary for the Project Reality mod I work on (for EA's BF2) AM2 6000+, 2gigs pc6400, x1950pro
Laptop : came with Vista Home Basic. Lasted 10 days before I got frustrated at slowness and installed XP Pro SP2 and now it is 60% faster (Dell Inspiron 1520, celeron M 1.86ghz, 1gig ram, onboard graphics)
Okay, I've shifted since the poll. My gaming box still has XP on it as that's where the games my kids play are at.
I have a laptop I use for general browing/IM/schoolwork, but after I got hit by that annoying XPantiviruspro2010 malware (is the new variant called XPdefender?) I had to reinstall and that's when I tried Ubuntu, had a load of problems with flash video and a girl I know directed me to Linux Mint 7 which comes all equipped on a basic install. It was problematic with full screen flash video (tv catchup sites sucked) and shortly Linux Mint 8 came out coinciding with a new Flash build and I was all sorted.
In two weeks the next version of Ubuntu comes out. A couple of weeks later hopefully Mint 9 will come out (it's a community version of Ubuntu) and then we will see drastic improvements and apart from gaming and video editing, it will be a Windows beater. We're going to be looking at 10 second boot times from after the BIOS screens and OS identification part of boot up. It is 99.9% secure right now as virus/malware writers aren't aiming their software at Linux. and the system of program installation means that anything trying to install will popup messages asking you to do something.
Virtually all software for Linux is free and if you always use the built in software program to install stuff from the Ubuntu led repositories, there is virtually no chance you'll get anything nasty. You won't have access to the latest builds of software, but you get a very family friendly secure system. No windows registry either so unlikely your OS will corrupt (XP users will nod their heads here at 6 monthly installs when their OS starts slowing down) and whlst Ubuntu generally needs a modern computer to run on (my laptop is a celeron 1.86ghz, 1gig ram, onboard graphics) you end up with an OS that runs faster than Windows. Although I can't turn the desktop effects on or I start to slow down (most effects are pretty but have no serious day to day function tbh)
The desktop for Ubuntu is running Gnome 2. You need to know that Gnome 3 will be a radical change from the 2D desktops we currently use. You can read up on it if you look online. We don't think the new OS from M$ will be like we've seen either. They know they are reaching a turning point, but Ubuntu will get there first.