I have recently received a new pc and would like to install Ubuntu on my old one.
Can I simply install it and write over the old hard drive, or do I need to do something special?
If you do not plan to keep/transfer the information from your old hard drive then you can simply do the installation and overwrite the hard drive, there is nothing you have to do beforehand to prepare the hard drive.
If however there is information on the old hard drive that you would like to keep then I would suggest you backup the contents of that old hard drive before you do the installation.
YES, you can just install it.
Try installing Ubuntu 12.04 LTS.
If you have an issue (while installing), this is the forum for help ..