I would NEVER install a brand new operating system over an old one. The reason is that any of the instabilities will be carried across. Always do a clean install. Lets face it if you got a virus you would end up re-formatting your harddrive and re-installing everything again anyway. It is also a good opportunity to do a spring-clean of your harddisk and remove the stuff you don't use.
I always keep my data on a separate partition so reinstalling the Operating system means I don't even have to back up the data.