They require no partioning of the hard drive.

They run like a normal Windows app, either booting to a dos prompt or running 'under' Windows.

The advantage of this is:

The biggest advantage is it allows you to try Linux and have to option of removing and going back to your original setup easily. You can just delete the Linux folders or uninstal it using the 'add/remove software' in the control panel.

Theres no tricky partioning, my partical distribution required me to make a boot disk and run a program called FIPS. After creating a partion I deleted it using FDISK, Windows now see's my 10gig hard drive as a 7 gig. However the other 3 gigs are formatted for Linux and I can access the other 7gig Windows partition under it. It makes sharing Linux files tricky when running Windows. I usually just save files I want to access under either system on my Windows partition.

The down side of this:

If your system crashes you could loose both Linux and Windows files. Linux usually put the OS in one partion and your personall files in another. If it crashes you may loose your work but the actually OS partion id fine and will stil boot.

If the distribution runs under Windows you'l need a powerful machine and it wil run slowly due to you running two OS's.

Even if the distribution runs as an app that boots you to a Dos prompt, it will still be just as effective to use a boot manager such as LILO that lets you select which OS to start.

The distributions I know of are: