SP 2 Can't install Access denied XP Home

I have upgraded my mobo, and I'm trying to install SP2 and I get part way through and access denied and roll back. I have completed Microsoft KB 873148 which is sorting out the key that I didn't have permission on and ticking allow and read. Also did the verbose part. But still I can't install I'm not sure if I have WI Installed but SP2 comes with WI.My new Mobo Is Asrock 939 dual-vista with Athlon dual CPU. I have unistalled all of Norton spy and antivirus, and I have run SymNRT.exe which is a Norton clear up Program. Can anybody help with this problem PLEASE

 

Hi Timw, I have looked in the bios but I couldn't see any ref to virus protection to enable or disable.

 

I have noticed that my CPU is at 50% when Idle. I don't know if this is relevant? I'l try your suggestion and report back

 

Timw After the reinstall I will post back the results