If you are upgrading to the latest Version 5 (5.88), which is probably what you should do if you are still running XP, then you can simply install over the old version.
If you are upgrading to Version 6, I'd uninstall Version 5 first to be on the safe side, and use a recent nightly build, rather than the (not very) stable Build 1376. If you then decide that you still prefer Version 5, you definitely need to uninstall first before reinstalling.
Hope this helps.
David