You have a 1TB hard drive, split into 3 partitions, so it looks to you like you have 3 discs.
Your main disc is c: On there are all your programs and data.
Disc D: is created automatically by HP and contains information needed to restore your laptop if things go wrong with it. DO NOT MESS WITH THIS DISC.
Disc E: is created automatically by HP and contains some HP tools. This is a very small disc and should not be messed about with.
Your C; disc is quite (more than most people have) and is largely empty, so you have plenty of space to load more programs and data.
As hc4361 says, you would be wise to buy an external hard disc and regularly back-up all your important data to it. If your hard disc fails you will lose all data, photos etc that you do not have in a second place.
You should also create yourself a set of restore discs (CDs, DVDs or a hard drive); if your hard drive does fail you can use these to regenerate your system on a new hard drive. That's your system, NOT your data. If you go into "Control Panel > Backup and restore" the system should lead you by the hand into creating these discs.
If you have windows 7 you can actually create a copy of your system on a second, external hard drive and restore onto that if necessary. That restores system AND programs. I know nothing about Windows 8.