I personally installed virtual box in ubuntu and then installed rhel 6. Virtual box is freely available.
For installing virtual box in ubuntu give command
apt-get install virtualbox
Then after starting virtual box, create a new virtual machine inside it by selecting the os as linux and then giving approximate maximum size of the disc to be 10gb. Minimum 6.5 gb will be required for installing rhel 6. After that point to .iso file of rhel6 and keep everything else default.
After that power on that virtual machine and start installing rhel. And after that you can install rhel just like installing it on physical machine and without worrying about corrupting or losing your actual physical machine.
Steps for installing rhel 6 ( Same for vmware and virtualbox )
1) Select Install or upgrade option.
2)Then keep other things default till you are asked for device where you need to select cd/dvd and skip checking of the external device.
3)Then warning comes where you should select "Install anyway"
4)Then again you will see another warning where you have to select "Reinitialize all".
5)Do simple steps as indicated till you reach where you need to select custom layout. Now make one partition of 6.5gb, mount point as "/" and "ext4" partition. Then format it.
6)Then select "Software Development Workstation software set" for installing and click next.
7)After that progress bar comes where you just need to allow things to happen on its own. Finally reboot.
8)Finally you are done and now rhel 6 starts as usual.