I thoroughly recommend Linux Mint 9 Main Edition which is built upon Ubuntu 10.04 Its easy to install and easy to use plus it comes with much of the software you are likely to need preinstalled
Linux Mint 9 Download
The Perfect Desktop - Linux Mint 9 (Isadora)
Linux Mint 9 User Guide Download pdf.
Also worth considering is Linux Mint Debian (slightly more advanced than the Ubuntu based release)
Linux Mint Debian Download
Alternatively install Linux Mint as a dual-boot with Windows using mint4win without partitioning mint4win works the same as Wubi as described below (not available on Linux Mint Debian)
You keep Windows as it is, mint4win only adds an extra option to boot into Linux Mint. mint4win does not require you to modify the partitions of your PC, or to use a different bootloader, and does not install special drivers. It works just like any other application.
mint4win keeps most of the files in one folder, and if you do not like it, you can simply uninstall it as any other application.
Boot in to windows insert the Linux Mint LiveCD and you will offered the option of installing inside windows which is where the mint4win installer comes in, you will be asked how many gigabytes you wish to allocate to Linux Mint (I recommend 8gb) then you set a password for your installation then click install and thats it.
Once Linux Mint is fully installed upon starting your PC you will be given a choice of which operating system you want to use Windows or Linux Mint
You download the ISO. image of Linux Mint 9 or Linux Mint Debian then you need to create a Bootable LiveCD/DVD for installation
Linux Mint 9 and Linux Mint Debian can also be run direct from the LiveCD/DVD from Booting up without touching your Hard Drive