This is a bit of a grey area where consumer rights are concerned. You buy the physical computer but the installed software/operating system usually has terms and conditions to which you must agree.
PC World probably have a clause in their extended warrantee to the effect that it only covers fixing faults in the physical parts of the computer and restoring software that was originally supplied with the computer, or installed and set up by them.
As a computer user you are free to choose from a huge variety of operating systems and supporting programs. It would be unreasonable to expect PC World or any other supplier to be keen on providing support and roll back for anything other than the software originally supplied by the manufacturer at the time the computer was purchased.
A good general rule of thumb is that software updates should be sourced from the computer manufacturer and not directly from Microsoft. The version supplied direct from Microsoft is generic and should work with most machines, but there are certain parts that are specifically fine tuned and tailored by computer manufacturers to work with their specific hardware set up.
Hewlett Packard in my experience can be a bit slow rolling out tailored versions of Windows updates. That can be a bit frustrating, especially as the default settings of many programs look at the software developer sites rather than Hewlett Packard's support site for updates. That appears to be the source of most incompatibilities and problems that occured with my system.
Good luck with trying to get your computer fixed to your satisfaction. I fear it will become a long and tedious process for the reasons explained above. There is no reason that your computer should come back from repair in anything other than a pristine state. Greasy fingerprints and dirt should never be evident on the casing and are unacceptable.