It will be compatible as long as the laptop has SATA, and it will certainly be faster at some things, but for a computer that old most of the slowness you experience will probably be caused by old age (slow CPU and not enough RAM). Also, be aware that XP/Vista aren't SSD aware so they'll require a bit of tweaking (and you won't get TRIM, but even without it an SSD will be a lot faster).
But even a cheapo SSD will be way faster than an old laptop HD, I have one of those reviled Kingston V300 SSDNows in my laptop and one in my parents' computer, it's a night and day difference compared to a HD. If you get one now and it doesn't end up making a difference, you can just chuck it in your new computer when you end up getting one.