Very Slow Boot

Hi Guys,
My PC is having real problems booting. When I switch it on First thing I see is the GFX card info, then it switches to showing the m/board model and the CPU info, at this point it just sits there for about 5 minutes before it goes on to showing memory and detecting hard drives. It then boots into windows slower than normal. Windows runs fine once it's loaded.
The odd thing is that I will sometimes get 3 or 4 mornings in a row when it just boots up as normal.
Anyone got any ideas?