CSS Issues? Multiple Issues, please help

Hello everyone!

So I’ve been working on a website for my dad’s water filter company and I spent some time looking through templates and decided to pretty much design it myself from scratch with CSS and HTML. I took a couple elements from the codes of templates I found and modified them to make them work. I tested it on Firefox, Chrome, and IE on my own computer and, except that my embedded YouTube video didn’t show in IE, it looked fine.

Then someone from the company was looking it over for changes and on their computer they had a very odd error where the page appeared fine, then, as you scrolled down the display was all messed up. As you scrolled back up it affected the display on the top of the page that had previously looked fine. Then, if you waited a moment, the upper display fixed itself, but the bottom of the page when you scrolled down still looked terrible.

That is a minor problem compared to the next one, because it could very well just be the display on that one computer.

My second and larger problem is that I decided to test it on another computer just to see. It displayed terribly, as if the resolution was much smaller than it was. I designed it to fit within 800x600, but even on the 1024x768 it appeared incorrect, and I upped the display resolution on that computer and it still displayed incorrectly. The only difference I can think of is that the OS on that computer is XP (I believe) whereas I created the site on Vista 32.

If anyone can help me I would REALLY, REALLY appreciate it. So I am attaching a screenshot of the 1st problem, a screenshot of the 2nd problem. You should be able to figure out the code from the live site. If you need more info than that, please let me know.

Here is the live website: CLICK HERE
Link to CSS