The general layout of the site is nice and clean. The links to begin the tutorial are a little confusing. I would either add a few more links to your "Quick Menu" from the start page, or I would include links in the content of the start page for people to click on that will take them to the beginning of your tutorials. It's easy to miss the "Let's Get Started" link in the left-menu.
A few questions - Is there supposed to be a gradient at the bottom of the quick menu? There is one , and it looks a little out of place.
Why did you squish your logo?

It's a good rule of thumb to never use sizing parameters within your img tags. If your logo is natively 350 x 200, then either let it display at 350 x 200 or resize it properly in an image editor. Using sizing parameters can slow down the loading time of the page, and it results in some strange looking graphics. If you do resize your logo in an image editor, please do so with the correct aspect ratio. If you want it to be no taller than 130, then you need to resize it to 65% of its original size, which is 228 x 130. If you want it to be 350 pixels wide, then you need to leave it at 350 x 200.
As I said, though, the site has a nice, clean, professional look to it. I would go back through and proofread your content again (or better yet, have a friend or relative do so), though, as I found some simple grammar and spelling mistakes while glossing over it.
EDIT - BTW - I am viewing your site in Mozilla Firefox 0.9.2 on a Windows PC.