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.
You usually need to uninstall v5 before installing v6. The default behaviour is that v5 and v6 installs to the same folders. When v6 is installed over v5, the v6 installer doesn't detect v5 (simply because v5 uses Inno, v6 uses MSI) and v5 registry entries are left behind, giving some users strange errors (notably, the unknown parameter '-hide' problem.)