Where to start?

• sidebar - font colors are hard to see.
• using images as titles or headings might not be good for SEO and accessibility
• some pages have no content http://www.stereoandalarm.co.uk/index.php?id=10&click=3
• because the site is white on black, there may be an issue of printing (with different font colors and all)

I want to add to Boon's answer. I agree with the first three points he makes.

I don't necessarily agree with the last point - not every site on the planet is made to be printed, nor does every site need to be printed, and this site probably fits into that case. Also, if you DO have a site that does white on black (or in general - light on dark) you can still make it print fine if you feel that printing is something that is a desire. You can use a print CSS stylesheet to change the colors to make it work better for printing.

My additional issues:

• The logo looks like it was scanned in off of a document that was faxed multiple times first. A clean logo can do wonders. It also doesn't really convey what they say are the services that they provide.
• Animated gif on the home page - are we back in 1997?
• http://www.stereoandalarm.co.uk/index.php?id=3&click=3 - I don't even know where to start with what is wrong on this page.
• Most importantly, there doesn't seem to be any content strategy. A bunch of the links go to pages with nothing but a title and sometimes an image that doesn't really convey any meaning to anyone. This page is absolutely worthless: http://www.stereoandalarm.co.uk/index.php?id=14&click=3

In addition to the comments already raised, it's not a professional looking site at all. (Not to mention the obvious typo with the spelling of 'professional' on the home page.)

Delivering a quality web experience is important as it is often a customer's first impression of the business. There's nothing here to draw me in and make me want to spend time on the site.

Here are some examples of competitors that have simpler sites that are less chaotic:

• Car Audio Direct
• John Kleis

The second example, is still a work in progress. However, it's laid out well and doesn't put me off visiting his store. The black logo also happens to be in keeping with his storefront, so it helps to establish his 'brand' as it were.