Microsoft says my Win7 Ultimate is an illegal copy!

I bought a used Acer Aspire with Win 7 Ultimate pre-installed.
I couldn't get it to work properly (and couldn't even delete
the pornographic screen-savers installed by the previous owner!).

So I retrieved the activation code with Belarc Advisor and installed
from the iso from Microsoft's website.

It all worked perfectly but after not using the Acer for a couple
of months when I next turned it on I got a message which claims
that the installed Win 7 Ultimate is not a legal copy.

How can I fix this?

Well, I don't know what an activator is and as
for the activation key it's the one returned by
Belarc Adviser so what could be wrong with it?

I doubt Acer would install an illegal copy of Win.
Why would they?

I don't need Ultimate? In fact I don't need Win 7
except that a number of programmes need a 64-bit
OS. Otherwise I'm happy with XP.

Is there a 64-bit XP? Now that's one I'd pay money for.

AddRAM

For the first time I read the "stickers" on the front of
the computer and it says "Windows 7 Home Premium" so
it seems that the previous owner installed an illegal copy
of Ultimate.

So can I assume that the activation code I retrieved will
be OK if I install Home Premium?

Ultimate is too complicated for me anyway so it'd be no loss.

Thanks for your reply.

As ThrashZone mentioned, download the tool, run it and under Win 7, choose Win 7 HP OEM.
Burn a DVD or install it on a USB Flash disk with Rufus - Create bootable USB drives the easy way
Do a Clean install Clean Install Windows 7
Use the COA key to activate.
For the updates MS releases SP2 for Windows 7