Question : Unable to install SP3 on Windows XP PC-Access Denied error

I have two PCs that I cannot install SP3.  The install runs all the way through to the end and returns and error that "Access is denied".  I searched Microsoft support website but could not find a solution.  

Answer : Unable to install SP3 on Windows XP PC-Access Denied error

Here is the answer to your issue:

http://support.microsoft.com/kb/949377

Hope this helps and good luck!