The first thing I notice when viewing the homepage, is all of the links in the navigation...then all the links under it. Maybe try to compact it down a bit, so there isn't too much stuff for the navigation.
- The computer systems page...with the drop downs. That could be improved. As I have looked at all of the drop down menus there, and there are only like 1-3 options other than the 'View Product' option. It would look better if you have links instead of these, and make the boxes slightly bigger. It would make for much easier finding of specific things.
- CSS. I would have had a look at your CSS, had you not have four different CSS files in the source code...why not just have one or maybe two...not four.
- Validation.
76 errors.
Not a bad concept for the design, but still needs a lot of work.