Being unemployed is probably the best time to start up a business - you have the time, and you should have the motivation to get it to work (if you need the income).
It just depends on what kind of business it is. If it is a low cost set up then you have little to lose - just don't get yourself into debt.
Devise a business plan (even if you're not borrowing) just to get an idea of figures and objectives. It is actually harder to get something off the ground when you already have a full time job, so make the most of the time but use it wisely.