[Kamikaze Chinchilla]

You also have to take into consideration the resources your computer has. Windows Vista and 7 are huge resource hogs compared to the previous versions and will make your life a living hell if you try to install those on an old computer. Double check to see if your computer can handle that OS (eg CPU, RAM, video+audio cards, disk space [especially disk space cuz Windows updates for the newer OS's are a *insert bad word here*...take a huge chunk out of your space])

But if this is just a startup problem, my guess is that you're probably going to have to get a registry cleaner/fixer and set up your computer so it doesn't load some of your current/unnecessary startup items. If that doesn't work, just reinstall the current OS. Though my guess is that you have too many startup items.