I would never buy an off the shelf computer again.The one I am using now is custom built and cost me almost �200 less than the shop bought one we got 18 months ago.
There isn't really a right or wrong time of the year to buy a computer if it is custom built,but I would wait for the January sales for all the big retailers. First of all you need to decide what you want to use the computer for,for example:if you want to play games on it you will need more RAM,a better processor and better graphics card than if you just want it for surfing the net and normal home computing. You also need to decide which operating system you wish to use.Most new PC's come with VISTA now,I have VISTA and love it but many people hate it and would prefer to still be using XP.