Boot Problem: Diag. Code – D is yellow

I just installed Win XP; now my os will not boot. I noticed there are 3-green lights & 1(D)- yellow light on the Diagnostic Code Lights.

The system just hangs at the logo when I start the computer. I purchased & installed a new Hard Drive and memory card -(128MB). This was not the problem because I still can not get past the logo while booting.

Can someone please offer any advice?

Thanks,
Jill