Hi, my first impressions upon viewing this site (Firefox, 1280x1024) was that there is an abundance of Times New Roman or similar font. This is a serif font, which looks better in print than it does on a web site. To me, that's the biggest change you could make to add professionalism. Plus it's really easy to change that. It will just give it a more polished look if you use verdana, arial, tahoma etc.
Actually, as I look further I have noticed that you do use a few different fonts in different places. For example, the Testimonials box - the title, main testimonial and date and the one under it are in varying fonts and sizes and this definitely detracts from the consistency of the site. By all means, vary the sizes but it's better to keep to one typeface throughout the site imo.
Regarding the header - the logo doesn't really look too professional either - i feel it falls into the category of 'anybody could do that!', i don't mean to offend you with that comment but to me it just looks like a gradient/drop shadow creation.
The actual layout of the content is pretty good. Again, it will look even better if you convert it all to a better font. Hopefully that's in ur style sheet so it would be simple to change.
Horizontal scroll-bars at 1024x768 also, at least on my machine. This is quite important as on 800x600 it's missing off a whole section of your site!
When you make a site that is selling something, your design needs to make the potential customers feel comfortable and that they can trust your company. This is why your site needs a total face lift, because at the moment I don't think your hits will be equating to sales.