Not aware of any 'cover systems' for whole boats - probably something you'll need to get made up specifically for your boat by a sailmaker or upholsterer.
If you get one made I recommend a heavy canvas one, held down with 5 litre bottles of water at each seam (every 2'). You'll need to arrange a 'ridge pole'; either your boom / mast in crutches or something specific.
Alternatively, use your mast as a ridge pole, and get a very large tarpualin (or two).
Whatever way you go, keep an eye on where the cover goes of the gunwales, as there is likely to be some chafe of the brightwork at this point.
Here at falmouth there are boats in the mariner that have a plastic cover shrink wrapped all over them,very big boats as well, don,t know if they can be used twice though
I use a system that consists of an aluminium frame that sits on the deck, which is then covered to form a 'tent' over the boat. The canvas is then held in place with old 4 liter bottles of screenwash.
These are pretty much a necessity in Sweden because of the snow. Although expensive, they do give excellent protection and allow you to work on the boat with the tent in place, which is great when it's raining.
Of course this is assuming the mast is down, which I have to do every year anyway due to some idiot putting an overhead powered train track between the sea and my wintering field.
You don't say anywhere how big your boat is, or whether you need to cover the whole boat or just the cockpit and hatches. We use a 2 or 3-man dome tent (picked up for about £25 in the end of season camping shop sales) which covers the whole cockpit and the main hatch. The forehatch has its own canvas cover.
Boat is 34 foot long, and will have to be covered completely. I like the look of SvenglishTommy's system, and will endeavour to find a manufacturer of this type in the UK.