To backup all important information, check the hardware for xp compability, check drivers for the hardware which are compatible with XP.
Windows should be installed after sheathing and before siding.
Prepare everything you need
Check the HCL
Yes, you should always be sterile before performing any experiment as to not add any variables.
back up all the date
take a deep breath a concentrate.
Probably an hour before exercising or performing for the same reason you wait an hour after eating to swim. Being too active too soon after eating can make you throw up.
Ensure a complete back up of data exists
Before you install new hardware Before you update drivers
usaully you need to wait 48-72 hrs before winding window down, but the people who tinted the windows will tell you
Unless you do not want to have to install windows 8 onto your computer, there is absolutely no reason to wait.
You should buy an upgrade version of Windows Vista however your computer if running XP is likely not to be powerful enough to run windows vista. You should run Microsoft Windows Vista Upgrade Advisor before upgrading as this will show you any problems your computer may have running Vista.