Installed 120 gig HD, Computer says I only have 15 gig.
If this is an empty drive, go into disk manager and remove all partitions for this disk and re-partition the drive. If there is data on the drive, move it to another drive BEFORE you do this.
If it still shows 15 gig, there may be a jumper on the drive set for compatibility mode. You can find the settings at the manufacturer web site.