Well damn, I typed up a post earlier today but my phone must have eaten it.
First, specs are going to be important on what it can run.
Secondly, what are you doing? If surfing APS and facebok are about the extent of your internet needs run a good AV software and call it a day.
If you are doing more though, I wouldn't use an unsupported XP. Check specs, see if it can run 7 or 8.1. If not, see if a reasonable upgrade can get you there, hundred bucks for new memory is going to be cheaper obviously than a new computer.
If not and you can swing it, there's always the upgrade option. The average tasks I can think of for an 8 years old XP machine can probably handled easily by a low cost 8.1 PC option.