Trohbock, list your setup, mainly hardware, and perhaps we can do more.
In the meantime, off the top of my head:
- if an older mobo (Via MVP3 chipset?) you would need more mobo-based cache memory (L2, 2meg, I believe) to be able to work with 512meg of system memory. Otherwise, symtoms are exactly what you describe.
- Check the memory in another machine.
- If you mean 512meg in one module, even some recent mobos balk at it.
- Along the same lines, some of the best-priced memory out there may be compatible only with several Athlon mobos but not with Pentium/Celery ones.