Installing windows 8

Hi!

I have bought the windows 8 as an upgrade to my old windows xp on my old Dell Dimension 5000.
But I have a problem, the only place I get to when I am turning the computer on is error code 0x00000050. I have read on the internet and I know how to solve it, but my problem is that I cannot get to safe mode. I have been trying f8 more than a thousand times and all of the other options there but I get nowhere!!
Please help ...