Two things come to mind. What memory does it have? Even if there isn't much stored on the PC a lot of memory is needed to run the processes involved in surfing, updates, flash player, etc. Can I suggest that you have a look at this site which will tell you what memory there is on the PC and what it could be upgraded to. You will need to do this on your parents' PC as it will be scanned.
The second thing is that it may have picked up a virus so perhaps running something like malwarebytes may help.
If you have CC cleaner installed you could run the cleaner to clear any temporary internet files. This could also be used to uninstall any programmes that are not being used.