Welcome to askubuntu!
Ubuntu can run nicely on older PCs. If you need something even lighter, you can try out different flavours, including very lightweight ones, such as Xubuntu or Lubuntu.
It's not very hard to set up dual-boot, i recommend you to follow the steps on this arcticle.
If you need any help, feel free to ask!
Regarding the release, you can chose between a regular release and a LTS one, both have their pros and cons.
Regular: Released every 6 months and Supported for 9 months, they're a great choice if you want to always get the latest features and packages.
The current latest release is (At the time of writting) 15.10 (Wily Werewolf).
LTS: Released every 2 years, they are Supported during 5 years. They're the best choice if what you're looking for is stability.
The current latest LTS release is (At the time of writting) 14.04.3 (Trusty Tahr).
If you want to read more about the different releases, here's a few links:
https://wiki.ubuntu.com/Releases
http://www.ubuntu.com/info/release-end-of-life
PS: If my post answered your question, please remember to mark it as the accepted answer.