Go to BIOS, you can find out how by reading your computer manual. It is done by striking one of the f keys (ususlly 2 or 12) repeatedly during boot. In BIOS change the booting order from Hard Drive to CD/DVD. Turn off your computer, put DVD in drive and boot up. Follow instructions on the screen.
When you get to the screen asking for custom or upgrade, choose the advanced option. Format in that option, then choose custom install. Do not give your key number until you are all done. Make sure you uncheck the box that asks to activate automatically.
When done, install your programs. Then try if for up to 30 days. If satisfied, then activate.
Note, You have to reinstall all of your prgrams and you must save everything else like documents etc. You can make it easy by using windows easy transfer, found on the MS site.
Do not forget to back everything up before you start. We never know what will happen.
Good luck