Personally I prefer a clean install. Theres less problems that way. But others may disagree with me.
If its an upgrade disk you have make sure you have your other disk handy ie Win98 or whatever because to install the upgrade it will be necessary to put it in to authenticate you own an original operating system and allow you to use the upgrade disk.
Make sure you have the motherboard driver disk as well as any other disk you might need such as modem, graphics card, printers etc. Back up any files you want to keep and write down websites you like and any emails addresses for reinstalling later.
Lastly can I ask what speed the PC is and how much memory you have as if its an older PC it will slow things down so much you may not consider it acceptable.