If you can't find a CPU Compatibility list on their site, ask their customer service. The three possible outcomes:
1. It can't be done.
2. A list of CPU's it can be done with, but it needs a BIOS-update also.
3. A list of CPU's that it can be done with without a BIOS update.
I think #1 is the most likely.
Upgrading the HDD from 160 to 250 GB shouldn't make a difference. For a difference you need an SSD.
But RAM from 1 to 4 GB should have made a difference, depending on how and what you measure and what you run.
I just Upgraded one of my Computers. An older Compaq Presario.
I Upgraded it from a 160 GB HDD to a 250GB HDD and also upgraded it from a 1GB Ram to a 4GB Ram.
I installed a Windows 7 Pro OS on it. Its working OK but not any faster than before.
Could it be the Processor?
I notice the cooling fan runs a lot at different speeds depending on the process under way.
Can I upgrade this unit to a Dual Core Processor?
Would that help it be any Faster. Its Really Slow.
Any Suggestions or Input would be Appreciated..
It now has an Intel Celeron 360 SL9KK 3.46GHZ/512/533/06 L6460432 installed in it.

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