I believe nothing is secure. Even you have the safest OS in the world, if you gave your private keys then security is pointless.
I actually use windows XP even outdated I feel secured because I don't keep my keys around.
Using Windows XP is really really not recommended though, as there are zero security updates for that operating system now. If you were to use Windows, probably at least use Windows 7 so you receive security updates somehow. Even though you're pretty secure on your browsing habits, it wouldn't hurt to use a secure operating system.