I don't think XP is eligible for the free upgrade, sure you aren't on Windows 7 when it starts?
There are  a few compatibility issue with specialised programs and some drivers e.g. your sound might not work without some tinkering.
If you aren't doing anything very technical with it, Windows 10 will be best yet at maintaining itself and being secure on its own.
But no need to feel presured into changing.
If you're likely to want a new machine anyway in 2 or 3 years, you might as well leave it and get the improved hardware optimised for W10 and guaranteed to work.