I have used Linux on my laptop (which is my only machine) since October 2001.
I use Debian (where else can I also have a complete portable development environment that matches my hosting environment).
If I were starting out, and wasn't technically minded, Linux isn't for the faint of heart when it comes to a desktop OS. There are things that still irritate me to this day regarding functionality, although, it is getting better.
If you were going to consider Linux as a desktop and you are moving from Windows, Xandros (as previously mentioned) is the way to go. Crisp and professional, it does what it needs to do and virtually emulates the windows look and feel. Xandros Pro includes Crossover Office and Crossover Plugin, which allow you to run native Windows Apps in Linux -- for the ultimate in compatibility with your existing Windows machine. Crossover does seem to have problems running Quickbooks and printing.
Then there's Linspire -- Lindows Renamed. Walmart sells it on their Linux PCs. Short and to the point, it works, its pretty simple. It is also based on Debian.
I haven't looked at Ubuntu personally, but, both of my tech guys rave about its 'ease of use'.
If you wanted to see Linux on your machine without having to format a disk, etc. Go to
http://www.knoppix.net/ and download Knoppix's LiveCD. Yes, you boot up on the CD and have a fully functioning version of (again Debian) Linux with KDE. No need to format the hard drive, no need to repartition, just slap in the CD and boot it up. Its fairly good at detecting most major brands of hardware.
Then there's MacOS/X which is based on FreeBSD.
You could also use FreeBSD, NetBSD or OpenBSD -- they are all unix varients and have fairly complete packages.
There aren't many applications out there that don't have a competing Linux/Unix version.
Microsoft Office ->
http://www.openoffice.org/ (also runs on windows)
Photoshop -> Gimp (some people say it is more powerful than Photoshop. I don't do graphics and could never figure out either PhotoShop or Gimp for doing simple stuff, so, I rotate pictures using the linux command line)
Dreamweaver can run under Crossover Office (I use vi for my webpage design so I don't know what other web tools there are)
Quicken -> gnuCash
Microsoft Project -> Planner (well, this isn't a great reproduction, but, it is getting better. Early versions it looked like they would press a button, see what changed and try to replicate the function. Don't see a change, well, leave the button there, it might be important.)
Internet Explorer -> Firefox (you can also run Firefox on windows, etc)
ACDSee -> Gtksee, gthumb (gtksee is almost a clone, although, I haven't used it since they did a rewrite)
WinAmp -> XMMS (virtual clone)
NetMeeting -> GnomeMeeting (yes, you can even videoconference with Microsoft Netmeeting)
Overall, you'll find that the more popular the hardware is that you have, the fewer issues you will have getting the drivers working. Your ATI or GForce card will probably work rather seamlessly. Your Taiwanese Hercules Clone with the special 4D megahyperaccellerator -- probably not so much.
Lilo or Grub (or any of the other bootloaders) will allow you to run multiple OSs. When I bought my last laptop, I resized the NTFS partition (so that I could keep WinXP on there if I ever needed it) and installed Debian. I also left 10gb for FreeBSD or whatever else I wanted to use. Grub comes up, defaults into linux, but, I can choose WinXP if I want.
My nephew's machine has Debian, WinXP and Win98 (one game he plays only works on Win98). It defaults into WinXP -- although, last summer he was in Linux quite a bit.