Windows 7 activation always come back after successful activation

I have a problem which MS Support have been unable to solve so far.  I have a very simple Media Centre PC which has been runing fine for over 12 months.  It had Vista Home Premium 32-bit on it.  I recently upgraded to Win 7 Home Premium and activated Windows ok.  After about a week the HDD came up with a SMART error and eventually failed.  I replaced the HDD, reinstalled Vista (and activated it) and reinstalled Win 7.  I activated it successfully but then noticed after a reboot it was still saying I had 3 days to activate.  I have done this about 3 times including the automated telephone system and it says successful but then after a roboot says it still needs activating in 3 days.

I spent hours on the phone to MS yesterday and tried other upgrade product keys.  Eventually we formatted the HDD and did a completely clean install with a new full product key they gave me.  Still the same, after a reboot it says 3 days to activate.

I've scanned the event logs and think it could be due to the event below in the Security-SPP Service startup.  I understand hardware changes will make it need reactivation, but nothing changes, just a reboot.  There is also a failure to start Null.sys and a couple of errors about atimkdag ('CPLIB :: General - Invalid Parameter' and 'Display is not active') but I'm assuming the H/W change event is the key.

The question is - how do I find out which piece of hardware it thinks is changing each time?  I'm assuming it maybe the HDD as that is the only thing that has changed from when it was stable.  Are there any utility which shows you the IDs which make up the Hash used in the activation?

I'm starting to run out of options but the old HDD should come back from Samsung soon so a complete reinstall to that HDD may be the only option.

The current spec is:
mb:    Gigabyte GA-MA78GM-S2H  (I also have the latest Catalyst drivers)
cpu:    AMD Athlon 4850e 2500MHz
mem: 2GB
hdd:   Samsung HD154UI (1.5TB).  The previous HDD was 1TB
dvd:   Plextor PX-608AL

The only other device is a Hauppauge Nova-T-500 TV Card, and a logitech kb & mouse.  It's a pretty simple setup.

Log Name: Application
Source: Microsoft-Windows-Security-SPP
Date: 20/01/2010 22:53:18
Event ID: 1040
Task Category: None
Level: Information
Keywords: Classic
User: N/A
Computer: MYHTPC
Description:
Hardware has changed from previous boot.
AppId=55c92734-d682-4d71-983e-d6ec3f16059f, SkuId=2e7d060d-4714-40f2-9896-1e4f15b612ad.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="Microsoft-Windows-Security-SPP" Guid="{E23B33B0-C8C9-472C-A5F9-F2BDFEA0F156}" EventSourceName="Software Protection Platform Service" />
<EventID Qualifiers="16384">1040</EventID>
<Version>0</Version>
<Level>4</Level>
<Task>0</Task>
<Opcode>0</Opcode>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2010-01-20T22:53:18.000000000Z" />
<EventRecordID>778</EventRecordID>
<Correlation />
<Execution ProcessID="0" ThreadID="0" />
<Channel>Application</Channel>
<Computer>MYHTPC</Computer>
<Security />
</System>
<EventData>
<Data>55c92734-d682-4d71-983e-d6ec3f16059f</Data>
<Data>2e7d060d-4714-40f2-9896-1e4f15b612ad</Data>
<Binary>600000000A0032000000010004000100010000000200000001000100010024946814A6801033E0A8188854F2749B1A14EA45F2AEB4C136B30D0001000101000201000301000401000500000601000700000801000901000A01000C01000000003200000001000400010001000000020000000100010001002494C45BA6801033E0A8188854F2749B1A14EA45F2AE521E36B3</Binary>
</EventData>
</Event>