1) find out if you have SATA or IDE.(SATA = really thin cable about as wide as a dime that connects to the motherboard, IDE = Wide Ribbon about as thick as a dollar bill that connects to the motherboard)
2) You may want to read your motherboard manual or list your motherboard model on here as you may have an IDE now but may be able to use SATA which is much better.
3) After you figure if you are going to use SATA or IDE figure out a good size, im guess a 500gb might do you justice.
4) Connect your new hard drive up, Format it so windows can see it using the software the hard drive comes with or downloading it from the manufacturers site.
5) Partition the drive, what i normally do on bigger drives is set a windows partition of about 40-80 gigs depending on how many programs and games me or the person im doing it for is going to be puttting on there. then i leave the rest for files.
6) Boot into windows off the old drive and cut and paste the files you want on to the file partition you set up.
7) Install windows onto the partition you set up for that. if you have a program like ghost where you can clone your partition it might save you trouble of installing every and just clone it to the new drive.