Ok first things first, PostNuke is the best CMS out there, it has a large team working on updates and patches constantly. It also also more flexible than many others. And also more modules are built with PostNuke in mind.
I know it may not look easy to design your own theme, as you require to know php. But there is any easy way to do it, trust me. All you need to do is go to Spidean and download their free version of AutoTheme. You will need to register before you will be able to access their downloads section. Once you've downloaded it. Unzip the module to you pc, then up load it to your modules main folder. Once you've done this go to your sites administration page and click on modules. Look for AutoTheme, if it's not there click on Regenerate. Once you've found it, Initialize then Activate it.
To install the cms on your pc, you will need to install php and MySQL on your pc. That can be tricky for a beginner but guess what there's also an easy anser to that. Just download easyPHP, it's a complete package that will install all required programes and configure it for you.
PLEASE NOT: this is only for home pc use, it is not for go live sites hosted on your own pc, as it will not be up to date with all the patches ect of the indervidual software themselves. But it is perfectly safe it is just to have a local server to work on your site.
Ok to install CMS packages, you first need to download the program usually in zip format. Then either unzip and upload, or if your host provides it, unzip onsite. Once you have done this, it's pretty straight forward. Usually the cms program will have either an install.php or setup.php. So all you do is type in the address of your site. Example http://www.mysite/install.php and it will do the rest for you, all you need to do is provide the information it requests, ie database name, password etc.
You also install on your pc in the same way. If you have any more questions, let me know.
Links below
http://www.easyphp.org/
http://www.spidean.com/
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UPDATE
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Ashley S I've turned on email contact in my account here, email me and I'll explain all.