I have an old pc running XP/SP3 which lives mainly in what SWMBO calls the family room. It doesn't get used much. It used to be our main pc but was dreadfully slow......even though I'd done all the usual speed-up things that Google, Mike, Milo, Mixture, et al tell us to do)
I then, additionally, had a series of slowness problems with BT internet service, and as BT engineers - (it felt like most of their Thames valley engineers were out at one point or another) couldn't do anything about the Internet service, I gave said pc to the local techie who took it away to investigate.
When it came back, the Internet service was noticeably better.
Strangely enough, everything else also seemed to be better/quicker aswell. It would actually do two things at once, which was novel.
I rang the techie and asked him what he had done, (before I told him it was quicker). He went on the defensive, until I told him it was quicker, not slower!! I told him that everything seemed to be better, and asked him what might have caused it.
He was pretty clear that he had done nothing to increase the speed of the pc, he'd only worked on the Internet connection, different ISP servers and various versions of Firefox, IE and etc.
So there was no obvious cause.
Two reasons occur to me for the change.
1. The little men who rush around inside the machine to make it work had had a rest, and were re-invigorated, and/or
2. Someone who knew what they were doing had used the pc for a change, and the pc felt loved and cossetted.