If your going to install as a primary you will first of all need to gather all information from your existing hard drive and back them up on dvds/ cds/flash driver or whatever media that you want. *notice you will have to reinstall your programs since most all do not support transfering from one computer to another*
*notice you will not need the Bios update* this is for issues only*
Next is the fun part ^.^
Turn your entire system off and remove the battery then remove the hard drive cover from the bottom and slide the new one in then reassemble.
You will now need to get your windows Vista installation cd and you should be able to reuse the product key on the bottom of your laptop for this.
Once the installation is finished you will be asked to activate your system but to do this you will need in install your drivers that you have downloaded previously one at a time. Keep in mind most all dell drivers extract to the C:/ then you will have to start the exe from there.
After that you should be back on the internet and will be able to activate your operating system now.
From that point on you will be able to reinstall your programs like you normally would.
Keep in mind you should allow some time for reinstalling all windows updates and basic programs such as Adobe Flash , Adobe Reader , and other things.